configure: improve the error message if gbm missing

This improves the error message in the case where libgbm is missing when
the KMS egl platform has been enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=706808

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 79ad416515b55ece8b9151876456555ff3fede2f)
Update .gitignore

(cherry picked from commit f2072ab07566461b849f60e4e347171fcc9a9877)

Conflicts:
	.gitignore
Add a test for cogl_framebuffer_push_path_clip

The test makes an L-shaped path that fills the whole framebuffer
except for the top right quadrant. It then clips to that and tries to
fill the framebuffer with a rectangle. Then it verifies that all of
the quadrants have the expected colour.

This is currently failing due to a bug in the primitive clipping.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 5404033220099b4a3c6cf32a0d269c4e98489fee)
Fix calculating the bounds when clipping from a primitive

When projecting the bounding rectangle of a primitive it was using the
modelview matrix twice instead of the modelview and projection
matrices so it was coming out with garbage.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 7e1f05c84013bb91248d691091df00f4f634c6cf)
cogl-framebuffer: Don't mark the clear clip dirty from the journal

This means that we can't cache the journal read_pixels optimization.

https://bugzilla.gnome.org/show_bug.cgi?id=719582

Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 550bae22d20c8d6d7cf1d090faa9c91619594077)
cogl-gst: video-sink: fix YV12/I420 support

(cherry picked from commit 637728dd89d51bc2809b4f0495c08f6f9eaeb8eb)
cogl-gst: video-sink: fix ayuv pipeline setup

(cherry picked from commit 002e2ec08bf2d98c30f08b813a652618589ac000)