backend-drm: allow to disable GBM modifiers
Allow to disable GBM modifiers at runtime using the environment variable WESTON_DISABLE_GBM_MODIFIERS.
This can be useful for debugging or when modifiers cause issues, e.g. in case modifiers use higher memory bandwidth and hence impose a lower resolution limit as it is the case with Intel Kaby Lake graphics.
Related to: #404 (closed) Signed-off-by: Stefan Agner stefan@agner.ch