Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

Fixes https://bugs.dolphin-emu.org/issues/13136.

This assertion is currently triggered by Pocoyo Racing (https://bugs.dolphin-emu.org/issues/13136).

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Migrates off the printf-based formatting where applicable.

Now that we've extracted all of the stateless functions that can be
hidden, it's time to make the index generator a regular class with
active data members.

This can just be a member that sits within the vertex manager base
class. By deglobalizing the state of the index generator we also get rid
of the wonky dual-initializing that was going on within the OpenGL
backend.

Since the renderer is always initialized before the vertex manager, we
now only call Init() once throughout the execution lifecycle.

We can use if constexpr with the template functions that pass in a
non-type template parameter, allowing the removal of branches that
aren't taken at compile time.

Compilers will generally do this by default, however, we now give a
gentle prodding to the compiler if this would otherwise not be the case.