Call glFlushRenderApple before running the core for a frame
This fixes #624.
Delaying the flush call until after running the core for a frame
caused glitches when the core changed screenSize while running.
Example: We set OpenGL to upload a videoBuffer and render it with
256x224, then run the core as it changes screenSize to 512x224.
Only then OpenGL uploads the videoBuffer as we call glFlushRenderApple
but it draws it with 256x224 texture coordinates.