clamp colors to avoid invalid colors
clamp colors to avoid invalid colors

previously a race condition may create invalid colors:
when
* in light-shading-mode
* the block is very bright
* and is higher than its west-neighbour
then the resulting light-factor is higher than 1 (making the color
brighter)
* in case the blockcolor is also very bright (e.g. Jack'o'Lantern) we
get an overflow

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