Damage flash: Reduce maximum alpha. Avoid fade overload

Flash alpha maximum is reduced from 180 to 127 to avoid player blindness
in combat. Flash alpha minimum is unchanged.
The 'damage_flash' value is now limited to max alpha, to avoid multiple
hits creating a huge value that causes flash to stay at maximum alpha
for a long period. Now alpha always starts to fade immediately after
taking damage.
Both problems can be seen in Minetest let's play videos.
Simplify and optimise some code.