Fix too bright player model glow textures causing wrong model colors

Fix too bright player model glow textures causing to display wrong colors, e.g. megaerebus with 'color 7' was yellow instead of green.
It fixes #2434 (closed)

The new fullbright glow textures are made by combining shirt and glow textures of the corresponding non-fullbright textures, with a brightness correction to make them about half bright (using the Gimp color filter "Exposure" that doesn't change the black areas).

The new fullbright shirt texture are now completely black, they must not overlap the glow textures otherwise the shirt color mixes with the glow color on the player model (I already fixed this issue only for the erebus fullbright player model).