gl-renderer: Add OpenGL ES 3 async output capture support

ReadPixels() implies a synchronous read back of the render buffer to
return pixel data. OpenGL ES 3 adds asynchronous read back support by
writing the pixel data into a dedicated buffer object. This commit
adds asynchronous read back support to the output capture code. It
spawns a read back request and schedules a timeout a few frames later
in order to store the pixels into the client SHM buffer.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>