Blender Git Loki

Fix: Make it easier to tell if Color Swatches are greyed out or not

This commit makes disabled/inactive color swatches look more obviously disabled
by halving their alpha values. It is necessary because the results of
ui_widget_color_disabled() (which is usually used when disabling widgets)
get overwritten by the color filling code here.

To avoid confusion, the checkerboard background (when the color genuinely has
alpha) will still only show in those cases. That is, when disabled, color swatches
won't show the checkerboard unless the color actually has an alpha component if it
wasn't disabled.