We need to *write* to the shared memory, not read from it.
cogl_texture_from_data() is read-only, it doesn't keep
the data in sync with the texture.
Instead, we have to call cogl_texture_get_data() ourselves
to sync manually.
Always use pageflipping, but avoid full repaint by copying back dirty
regions from front to back. Additionally, we dealy copying back until
we're ready to paint the new frame, so we can avoid copying areas that
will be repainted anyway.
This is the least amount of copying per frame we can get away with at all
and at the same time we don't have to worry about stalling the GPU on
synchronized blits since we always pageflip.
