Previously the layer combine on the test pipeline was set up to
replace the incoming color with the layer constant. This patch changes
it to sample the replacement color from a 1x1 texture instead. This
exposes a bug on the GLES2 backend where the vertex shader will be
generated with a size for cogl_tex_coord_out of 4 but the
corresponding declaration in the fragment shader will have n_layers,
which is 1. This makes the program fail to link and the test fails.
https://bugzilla.gnome.org/show_bug.cgi?id=662184
Reviewed-by: Robert Bragg <robert@linux.intel.com>
884647071c