This can be overriden with GL_IS_SLOW=0 at runtime
for systems with a fast GPU, though it doesn't make
much difference either way for non-KMSDRM cases.
For slower GPUs, this enables important scaling and
rendering optimizations that can substantially
improve performance (especially on fillrate-limited
GPUs common in SBCs).
Some platforms like the VisionFive 2 don't have working desktop GL,
so they must use GLES to be able to render at all. Qt doesn't try
to fall back if it fails to find a compatible EGL config, so we must
do so ourselves.
We can avoid disabling EGL entirely by forcing Qt to use GLES,
which is not impacted by the black window issue. This lets us
simplify back to EGL everywhere.
The latter will still match on a hybrid system when the iGPU's EGL
implementation is in use. This scenario works properly with EGL
rendering in Qt and SDL, so we want to allow that.
