dmabuf passthrough

Still to do:

• add subsurface stacking api
• make attaching regular textures to subsurfaces work
• test subsurface api without passthrough
• trigger a parent surface commit if a subsurface is attached or restacked - no need, the renderer takes care of it
• Add a version of diff that compares subsurface nodes as equal if the have the same subsurface and same size and just update subsurfaces if that comes up empty
• make renderer reorder subsurfaces
• trigger a full redraw if subsurfaces appear/disappear - the diffing code should handle it
• make tests with multiple GtkVideos (overlapping/nonoverlapping) - if we make the logo opaque, fishbowl covers this
• Add a 'subsurface mode' to the node diff code
• figure out if we want to keep the shm wl_buffer code or make it a debug-only feature
• figure out how to properly wait for the create roundtrip
• test and fix this on a hi-dpi system, and with fractional scales
• debugging support
  • [ ] add 'disable dmabuf' and 'disable passthrough' options in the inspector - we don't expose other behavior flags
  • add subsurface information to the inspector ?
• if the last subsurface does not have any drawing on top, raise it above the main surface in the end, so mutter can use it for scanout without second thoughts
• check for clips and don't do passthrough if the node isn't fully contained (should take care of rounded corners)
• [ ] make gtkgstmediastream negotiate and return dmabufs - deferred
• [ ] handle wayland priorities and tranches when sorting dmabuf formats - deferred
• [ ] add per-surface dmabuf format information - deferred
• [ ] figure out what to do about buffer pools - deferred

This implements #6132 (closed)