Better assignment of views into HW planes
This MR is a reopened version of !267 (merged), which adds support for the
zpos KMS plane property. Supporting this property allows Weston's KMS backend to support alpha-blended hardware planes, as well as to place 'overlay' planes underneath the KMS primary plane, and allow overlay planes to overlap each other.
When !267 (merged) was merged, the 'squash commits' button was erroneously selected, so it was reverted via !296 (merged). This MR only reapplies the changes which are already present in !267 (merged).
Please do not add any new discussion or commentary in this MR. If you want to follow up discussion or reference the
zpos support in the DRM/KMS backend, please use !267 (merged) to comment and reference.