Fix openGL mipmaps syncing problem on OSX

The mipmaps problem on OSX happened because we used different
openGL context for uploading textures data and rendering. With
this patch, uploading will happen in the same context, so
explicit syncing with glFinish is not needed anymore.

WARNING: this patch changes the way how we render the frames.
I have streamlined rendering of new uploaded frames and added
a FPS-based compressor before them (to avoid too much context
switches). In my opinion, the rendering became slightly faster,
but the painters might not agree :) So it needs a bit of
testing.

CC:kimageshop@kde.org