Support underlays in DRM backend
Submitted by Pekka Paalanen
Some hardware supports underlays instead of overlays: YUV hardware plane can only be beneath the (primary) RGB plane. Or perhaps the hardware has several planes and some or all are in fixed z-order. Any planes beneath RGB plane we use as the primary would be currently useless in Weston.
Implement support for detecting the cases where using an underlay is possible, beneficial, and which surfaces it would take. Using an underlay also requires the renderer to create a punch-through hole in the primary framebuffer by painting a region as fully transparent.
Support for atomic must land in Weston before this task can be started: