core: Convert_Scale: Limit conversions to 16 bits.

This limits the conversions on output to 16 bits in order not to cause
overflows.

The code is also cleaned up a little.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: core: Convert: Comment out gray in tests.

The upscaling is not precise by definition.

I will have to change the code to compute acceptable
delta given the value and input and output size in
order to make it work.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: Convert: Fix rounding error on CMYK to RGB.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>