This adds a public function to read pixels from a framebuffer into a
CoglBitmap. This replaces the internal function
_cogl_read_pixels_with_rowstride because a CoglBitmap contains a
rowstride so it can be used for the same purpose. A CoglBitmap already
has public API to make one that points to a CoglPixelBuffer so this
function can be used to read pixels into a PBO. It also avoids the
need to push the framebuffer on to the context's stack so it provides
a function which can be used in the 2.0 API after the stack is
removed.
Reviewed-by: Robert Bragg <robert@linux.intel.com>