This adds an extra test to test-offscreen then ensures the offscreen
framebuffer for a texture automatically flushes its journal in the
following three situations:
1. cogl_read_pixels is called immediately when the offscreen buffer is
current.
2. cogl_texture_get_data is called on the offscreen's texture
immediately after rendering to it.
3. The texture is rendered to the screen and immediately read back
with cogl_read_pixels.
Currently the 2nd situation fails.
https://bugzilla.gnome.org/show_bug.cgi?id=668913
Reviewed-by: Robert Bragg <robert@linux.intel.com>
f4a42fe988