Consider switching the API to RGBA `[]uint8` instead of `[]uint32`

[ske2004]

It will be simpler for the user to change each RGBA values, it will also remove a lot of redundant conversion code.
The only problem is that we use uint32 to specify colors in other places.

Related: #218
CC: @marekmaskarinec

[marekmaskarinec]

I'm not a fan of this solution. First of all it will make the api incinsistent as you mention. Secondly, it might end up with the user making the conversion umka side, which will be much slower than c side.

I could see a possible solution with making two variants of the function.