fix RGB16 getG() method

there was a typo that led to the return of wrong numbers when
asking a ColorRGB16 value for its (8bit) green value (getG()).
This led to very wrong results when blending colors.
add a test for converting RGB16 into its components

test upscaling 5/6bit RGB16 components to 8bit values
Merge pull request #250 from hwhw/fix_kr1040

fix RGB16 color components 8bit value calculation