compositor: specialised fragment shader for RGBX

compositor: specialised fragment shader for RGBX

Remove the weston_surface::blend attribute, which really meant that the
texture produced valid alpha values. This was used to override the opaque
region for RGBX surfaces, which produce undefined values for alpha.

Instead, compile a new shader especially for RGBX surfaces, that
hardcodes the sampled alpha as 1.0.

Before "compositor: optimize/simplify shaders" there was a 'vec4 opaque'
in the shaders, that would cause part of the texture to be forced to
alpha=1.0. Now that is gone, and we need this replacement.

To test: launch simple-shm, and use the super+alt+mousewheel combination
to make it transparent. It should not show a light cross over the window.

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