83dc468 broke the OSD message that shows the number of custom textures.
If Prefetch Custom Textures was disabled, the number would always be 0.

When using clang and libstdc++, compiling fails with these errors:

```
FAILED: [code=1] Source/Core/VideoCommon/CMakeFiles/videocommon.dir/PerformanceMetrics.cpp.o
/usr/sbin/ccache /usr/sbin/clang++ -DAUTOUPDATE=1 -DDATA_DIR=\"/usr/local/share/dolphin-emu/\" -DHAS_OPENGL -DHAS_VULKAN -DHAVE_CPPIPC -DHAVE_LIBSYSTEMD -DIMGUI_USER_CONFIG=\"imgui_user_config.h\" -DLZMA_API_STATIC -DOFF -DPUGIXML_NO_EXCEPTIONS -DRC_CLIENT_SUPPORTS_HASH -DSFML_STATIC -DSPNG_STATIC -DTINYGLTF_NOEXCEPTION -DTINYGLTF_NO_EXTERNAL_IMAGE -DTINYGLTF_USE_CPP14 -DUSE_ANALYTICS=1 -DUSE_MEMORYWATCHER=1 -DUSE_PIPES=1 -DUSE_RETRO_ACHIEVEMENTS -DZSTD_MULTITHREAD -D_ARCH_64=1 -D_DEFAULT_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_M_X86_64=1 -D__LIBUSB__ -D__STDC_CONSTANT_MACROS -D__STDC_LIMIT_MACROS -I/dolphin-local/Source/Core -I/dolphin-local/Externals/picojson -I/dolphin-local/build/release/x64/Source/Core -I/dolphin-local/Externals/enet/enet/include -I/dolphin-local/Externals/fmt/fmt/include -I/dolphin-local/Externals/mbedtls/include -I/dolphin-local/Externals/minizip-ng/minizip-ng -I/dolphin-local/Externals/liblzma/api -I/dolphin-local/Externals/zstd/zstd/build/cmake/../../lib -I/dolphin-local/Externals/libiconv/include -I/dolphin-local/build/release/x64/Externals/libiconv/lib -I/dolphin-local/Externals/libiconv/libcharset/include -I/dolphin-local/build/release/x64/Externals/libiconv/libcharset -I/dolphin-local/Externals/SFML/SFML/include -I/dolphin-local/Externals/bzip2/bzip2 -I/dolphin-local/Externals/expr/include -I/dolphin-local/Externals/hidapi/hidapi-src/hidapi -I/dolphin-local/Externals/pugixml/pugixml/src -I/dolphin-local/Externals/glslang/glslang/glslang/.. -I/dolphin-local/build/release/x64/include -I/dolphin-local/Externals/rcheevos -I/dolphin-local/Externals/libspng/libspng/spng -I/dolphin-local/Externals/xxhash/xxHash -I/dolphin-local/Externals/imgui -I/dolphin-local/Externals/imgui/imgui -I/dolphin-local/Externals/implot/implot -I/dolphin-local/Externals/glslang/glslang/SPIRV/.. -I/dolphin-local/Externals/tinygltf -isystem /dolphin-local/Externals/libusb/libusb/libusb -isystem /dolphin-local/Externals/glslang/glslang -isystem /dolphin-local/Externals/glslang/glslang/Public -isystem /dolphin-local/Externals/glslang -Qunused-arguments -fcolor-diagnostics -O3 -DNDEBUG -std=c++23 -msse2 -fdiagnostics-color -fno-strict-aliasing -fno-exceptions -fvisibility-inlines-hidden -fvisibility=hidden -fomit-frame-pointer -Wall -Wtype-limits -Wsign-compare -Wignored-qualifiers -Wuninitialized -Wshadow -Wshadow-field-in-constructor -Winit-self -Wmissing-declarations-Wmissing-variable-declarations -Werror=format -Wno-trigraphs -MD -MT Source/Core/VideoCommon/CMakeFiles/videocommon.dir/PerformanceMetrics.cpp.o -MF Source/Core/VideoCommon/CMakeFiles/videocommon.dir/PerformanceMetrics.cpp.o.d -o Source/Core/VideoCommon/CMakeFiles/videocommon.dir/PerformanceMetrics.cpp.o -c /dolphin-local/Source/Core/VideoCommon/PerformanceMetrics.cpp
In file included from /dolphin-local/Source/Core/VideoCommon/PerformanceMetrics.cpp:4:
/dolphin-local/Source/Core/VideoCommon/PerformanceMetrics.h:69:51: error: no matching constructor for initialization of 'std::atomic<FrameBufferSize>'
   69 |   std::atomic<FrameBufferSize> m_frame_buffer_size{};
      |                                                   ^~
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/16/../../../../include/c++/16/atomic:237:17: note: candidate constructor not viable: constraints not satisfied
  237 |       constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Tp>)
      |                 ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/16/../../../../include/c++/16/atomic:238:11: note: because 'is_default_constructible_v<PerformanceMetrics::FrameBufferSize>' evaluated to false
  238 |         requires is_default_constructible_v<_Tp>
      |                  ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/16/../../../../include/c++/16/atomic:246:7: note: candidate constructor not viable: requires 1 argument, but 0 were provided
  246 |       atomic(const atomic&) = delete;
      |       ^      ~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/16/../../../../include/c++/16/atomic:252:17: note: candidate constructor not viable: requires single argument '__i', but no arguments were provided
  252 |       constexpr atomic(_Tp __i) noexcept : _M_i(__i)
      |                 ^      ~~~~~~~
1 error generated.
```

This was tested and occurs on both Arch and Alpine Linux.

fb4ff3e put all the code for the show internal resolution setting right
next to the code for the show FPS setting, presumably because an earlier
version of that commit had them next to each other in the GUI. It makes
more conceptual sense to put the code next to the code for the show
statistics settings, matching what the GUI looks like now.

It looked a bit out of place wedged in between the FPS and VPS.

fb4ff3e added a statistics option to show the internal resolution, but
it just showed the total size of the EFB (which is always 640x528) times
the IR scale, so it didn't convey any useful information.

This commit instead makes the option use the size of the last XFB copy
(not multiplied by the IR scale), which changes based on the game's
rendering resolution.

This makes the struct 8 bytes smaller.

VideoCommon: Added option for showcasing internal resolution