Make DRM plane-assignment algorithm tolerant of more than one fullscreen opaque surface?

[Matt Hoosier]

Currently the DRM backend ends up picking the bottom-most view which meets
all the checks for eligibility, for direct scanout usage. If more than one
such view exists, we get a visual result counter to expectations -- it
should be the highest-sorted such view that the user sees.

The big loop in drm_assign_planes() that assigns views to planes iterates
top-down through the weston_layer's, and the particular way that the
results are progressively stored in variables means that if more than one
pass through the loop encounters a view which is fullscreen and opaque then
the last (bottom-most) of these passes is the one whose results are
preserved upon exit from the loop.

This normally isn't a problem because mostly only the desktop shell has a
notion of fullscreen surfaces. desktop-shell takes care (whether
intentionally or not I can't tell) that when running full-screen only the
logical topmost view is actively left in a visible weston_layer.

Is it informally expected that the shell must not allow more than one
fullscreen view at a time? Would there be interest in a patch to
drm_assign_planes() that adds a bit more awareness to the plane assignment
so that only the topmost fullscreen opaque view is picked for scanout?
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