Only scanout when the texture matches the display size

Otherwise drmModeAtomicCommit() in DrmOutput::doAtomicCommit() fails due
to unmatched buffer sizes.

While rendering continues working properly, this makes drm freak out and
try to go back to a previous state (see the test commit in
presentAtomically()) that in turn starts issuing screen events to every
process even though it's just to say the same thing. The fact that this
happens per frame makes the system unusable as soon as fullscreen starts
happening on a scaled display.

Another thing we could do is get EglGbmBackend::scanout() to take care
of the resizing.