Adds linux-dmabuf support to the pixman renderer. The main reason is so
clients can use GPU-accelerated rendering even if the compositor is
using the pixman renderer for whatever reason. This also allows the
pixman renderer to import dmabufs (only RGB/ARGB formats, not YUV for
now).
Since after this change all renderers store a DRM FD, I move the drm_fd
from the individual implementations to the shared wlr_renderer base.
This means that renderer_autocreate() can set pixman's drm_fd for it and
the pixman renderer doesn't have to know about DRM at all.
Originally based on
961edfe44e
Add wlr_pixman_buffer_get_current_image for wlr_pixman_renderer.
Add wlr_gles2_buffer_get_current_fbo for wlr_gles2_renderer.
Allow get the FBO/pixman_image_t, the compositor can be add some
action for FBO(for eg, attach a depth buffer), or without pixman
render to pixman_image_t(for eg, use QPainter of Qt instead of pixman).
