Incremental CRTC_* property changes

[emersion]

Leftover from #6 (comment).

If only CRTC_{X,Y,W,H} changes and the intersection between layers doesn't, we can re-use the previous alloc.

Note, on some Intel platforms sometimes putting a layer into a plane can succeed if the position coords are even (for some YUV formats). Let's not bother about this case.

[subdiff]

Something similar should hold for the alpha property. Once suitable plane has been found alpha should always be changable without the need to realloc.

[emersion]

The alpha optimization has been added in 9bfe58b ("Don't re-alloc when alpha changes").