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).
Material Conversion Plugins now ERR_FAIL if called on an unitialized material.
FileSystemDock no longer crashes if Conversion Plugin fails and returns a null ref.
