Makes sure libliftoff doesn't allocate a plane for the layer and doesn't crash.
References: 799f694587 ("Fix segfault when FB_ID isn't set")
References: 53a7bfebc9 ("Don't allocate planes for layers without a FB")
Compositors need to drive multiple connectors, each with its own vblank timings.
For each device, there's one separate rendering loop per output.
It's not possible to call liftoff_device_apply each time we want to submit a new
frame to one output, because this could touch another's output state, submitting
a new frame there in the process. When the other output will submit a new frame,
it'll get EBUSY (can't submit two frames without waiting for vblank).
Closes: https://github.com/emersion/libliftoff/issues/21
And restore the original zpos-4x-disjoint test. Depending on the constraints on
the last two layers, one branch of the layer zpos check or the other is taken.
Fixes: 8617ddfd05 ("Add intersection check for planes under the current one")
This test makes sure composited layers make layers underneath fall back to
composition too, in a transitive manner. That is, if plane A is under plane B
which is itself under plane C, plane A falling back to composition means that
both planes B and C fall back to composition too (even if C isn't directly
underneath A).
If a layer cannot be mapped to a plane, make it so all layers under it also fail
plane allocation so that they can be properly composited on the primary plane.
A test was incorrect and has been fixed. Another test had two possible solutions
and has been updated with the one libliftoff yields.
zpos-3x maps three layers to primary, cursor and one of the overlays planes.
zpos-4x-partial sets up 4 layers but is only able to find a plane for 3 layers,
because ordering between the two overlay planes is undefined.
When testing the library, the libdrm dependency is swapped with a mock libdrm.
The test sets up the mock libdrm's internal state and then runs the test.
