common/font: avoid rendering semi-transparent background twice

common/font: avoid rendering semi-transparent background twice

Fill with the background color first only if the background color is
opaque. This is necessary for subpixel rendering to work properly (it
does not work on top of transparency).

However, if the background color is not opaque, leave the buffer
unfilled (completely transparent) since the background is already
rendered by the scene element underneath. In this case we have to
disable subpixel rendering.

v2: use 0.999 alpha cutoff and fix CodeStyleCheck

Fixes: (the rest of) #1684