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...)

VideoBackends / VideoCommon: add support for specifying include files in shader code

VideoCommon: Remove old iLog field

Prior to 7854bd71098fe556ddc9839ad08d27fab7ecba33, this was used by the debugger for the OpenGL and D3D9 plugins to control logging (via PRIM_LOG and INFO_LOG/DEBUG_LOG in VideoCommon code; PRIM_LOG was changed in 77215fd27ccb7cdeb82fed24a7a8d4d4ad4c33a4), and also framedumping (removed in 64927a2f81fe64503060c949ead525995fd63a2a and 2d8515c0cf754806dc38222c4461053e12df1194), shader dumping (removed in 2d8515c0cf754806dc38222c4461053e12df1194 and this commit), and texture dumping (removed in 54aeec7a8ff6b6e5d050637b6cce8df6d9a8b633). Apart from shader dumping, all of these features have modern alternatives, and shader source code can be seen in RenderDoc if "Enable API Validation Layers" is checked (which also enables source attachment), so there's no point in keeping this around.