gbm.rs' lock_front_buffer calls has_free_buffers before calling the underling C lock_front_buffer function on the gbm surface. This has_free_buffers() check succeeds the first time but always fails after that.
However apart from this check, everything else appears to be working perfectly fine. Projects like weston, ksmcube, Qt, or mutter don't do this check beforehand either, so this patch works around gbm.rs by skipping this.
Unfortunately this requires going to gbm-sys directly and implementing the drm traits that require calling the various gbm_bo_get_* functions to retrieve the buffer details for posting to DRM.
This is the initial implementation based on DRM dumb buffers,
which cleans up various things at the same time:
- DRM output is separate from GBM and DRM dumb buffer displays.
The latter two re-use the former.
- The timer refresh for the Vulkan KHR Display rendering lives
now with the KHR display code, in hope to be replaced with a
better mechanism in the future.
What's missing still is support for partial updates as well as
rendering with our own software renderer.
