Removes RS as a dependency in `project_settings.cpp` (which was a bug,
`core` shouldn't include `servers`). This doesn't have a big impact on
incremental rebuild time by itself.
Also move helper `get_splash_stretched_screen_rect` to RenderingServerTypes.
A number of headers in the codebase included `rendering_server.h` just for
some enum definitions. This means that any change to `rendering_server.h` or
one of its dependencies would trigger a massive incremental rebuild.
With this change, we decouple a number of classes from `rendering_server.h`,
greatly speeding up incremental rebuilds for that area.
On my machine, this reduces incremental compilation time after an edit of
`rendering_server.h` by 60s (from 2m57s).
