We rename it flush_damage() as it's the point where we update our rendering
API source (eg, the gles2 texture) according to the accumulated damage,
if necessary.
We move the EGL and GLES2 output repaint code into a new gles2-render.c
file. The eglMakeCurrent, glViewPort, surface loop etc was duplicated
across all backends, but this patch moves it to a new file.