[bitmap] Fixes _cogl_bitmap_fallback_unpremult

[bitmap] Fixes _cogl_bitmap_fallback_unpremult

The _cogl_unpremult_alpha_{first,last} functions which
_cogl_bitmap_fallback_unpremult depends on were incorrectly casting each
of the byte components of a texel to a gulong and performing shifts as
if it were dealing with the whole texel.

It now just uses array indexing to access the byte components without
needing to cast or manually shift any bits around.

Even though we used to depend on unpremult whenever we used a
ClutterCairoTexture, clutter_cairo_texture_context_destroy had it's own
unpremult code which worked which is why this bug wouldn't have been noticed
before.