renderer/native: Allow forcing EGLStream backend
The GBM support in the NVIDIA driver is fairly new, and to make it easier to identify whether a problem encountered is related to using GBM instead of EGLStreams, add a debug environment variable to force using EGLStream instead of GBM.
To force using EGLStream instead of GBM, use
MUTTER_DEBUG_FORCE_EGL_STREAM=1
Related: #2045 (closed)
Untested so far.