Bypass eglMakeCurrent where possible.

This has the possibility to be very slow on some GPUs. A nicer alternative would
have been to fix this in QOpenGLContext, but with makeCurrent and updateContext
having been merged in Qt 5, makeCurrent is required every frame call, and thus
cannot be fixed there.

Change-Id: Ib17dbb3a1e4e89c60dfd4f12a55eeff353f9075f
Done-with: Carsten Munk <carsten.munk@jollamobile.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>