surface buffer cardinality and outputs

[Sylvain BERTRAND]

Well, if a toolkit wants to render the same GUI on outputs of
differents properties (those properties significant for actual
rendering), they will have to render several times, no other
choice to do things properly.

The other option would be to ignore those output properties, and
the compositor would manage something with an output agnostic
buffer.  In that case, we would remove the subpixel property from
the output to stay consistent (mode is used for shell surface
fullscreen).

Your proposal is middle ground: a set of buffers, and some
heuristics applied on their properties to combine those in the
compositor space. For simplicity, I dare to think one buffer for
one output is quite clear cut and easier to deal with.

Of course, there is also the brutal way: output becomes a
singleton, keeps its properties to let the toolkit render
"properly" (hopefully that would be significant), namely we would
drop support of several outputs.

-- 
Sylvain