Allow reverse PRIME with an explicit configuration
Some (POS or other embedded) deployments require separate X screens for fullscreen applications. This requires using an Xorg configuration file.
One such use case adds an extra USB DisplayLink device which is a dumb framebuffer. To reduce CPU usage for drawing, 3D backing is needed which can be provided by (reverse) PRIME.
Currently there are a few problems:
- RandR providers are always assigned to screen 0 instead of the ones they were assigned to in the configuration
- glamoregl refuses to be initialized on the llvmpipe driver which would be used for a framebuffer-only device.
This MR lifts these limitations and allows (reverse) PRIME to be set up using a configuration file.