Support underlays in DRM backend
Submitted by Pekka Paalanen @pq
Description
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.
Refererence: https://lists.freedesktop.org/archives/wayland-devel/2016-March/027441.html
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.
Ideas: https://lists.freedesktop.org/archives/wayland-devel/2016-March/027451.html
Support for atomic must land in Weston before this task can be started:
https://bugs.freedesktop.org/show_bug.cgi?id=83888