Code review comment for lp:~vanvugt/mir/fix-1339471

Qt/Qml apps render with premultiplied alpha by default. Cairo does premultiplied alpha by default too.

I think it's reasonable to demand (and document clearly) that surfaces with alpha passed to Mir are premultiplied.

Perhaps we could let the clients select per surface, if we find that important clients can't do premultiplied alpha. Even so, premultiplied alpha would a reasonable default.