This adds the necessary fuzz to image matching to let GL-renderer pass.
The difference is due to rounding. weston-test-desktop-shell.c uses

weston_surface_set_color(dts->background_surface, 0.16, 0.32, 0.48, 1.);

to set the background color. Pixman-renderer will truncate those to uint8, but
GL-renderer seems to round instead, which causes the +1 in background color
channel values.

	0.16 * 255 = 40.8
	0.32 * 255 = 81.6
	0.48 * 255 = 122.4

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>