[cairo] add glClear

[Adam Jackson]

On Wed, 2013-02-20 at 20:19 +0000, Henry (Yu) Song - SISA wrote:

> Do you think it is a good idea to add glClear right after
> glXSwapBuffers/eglSwapBuffers in cairo.  The reason is I know some
> (might be all?) drivers use glClear as an indicator to start a new
> frame and thus reset some buffers, for example, buffers in glDraw().  

I might hesitate.  cogl goes out of its way to track the regions it's
about to damage precisely because it can be a win to reuse the preserved
bits of the back buffer rather than blow them away.

- ajax