compositor: paint opaque regions with RGBX shader

compositor: paint opaque regions with RGBX shader

weston_surface_draw() is restructured so that it will always use the
RGBX shader for opaque regions, if the surface is assigned the RGBA
shader.

Previously for opaque regions, we simply assumed, that the texture alpha
would be 1.0. If it was not (which really is an application bug), the
region would be misrendered. The RGBX shader forces the texture alpha to
1.0.

Xwayland surfaces may have bad alpha data in the opaque client area. If
blending was enabled, the bad alpha would be used with the RGBA shader.
This patch fixes rendering opaque xwayland windows with full-surface
alpha applied.

Test case: xterm, with full-surface alpha one step below 1.0. Before,
black text was fully transparent, now it is correctly only slightly
transparent.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>