[rtextures] advance k in LoadImageColors

[brccabral]

Some formats are not advancing k to get pixels values

[raysan5]

@brccabral Thanks for the review!

SIDE NOTE: I added a TODO with a possible improvement: instead of always returning PIXELFORMAT_UNCOMPRESSED_GRAYSCALE, it could return other single-channel formats like PIXELFORMAT_UNCOMPRESSED_R16 and PIXELFORMAT_UNCOMPRESSED_R32 when the image provided to extract channel is a 16bit/channel or 32bit/channel data. So, no data is lost in those cases (and it's up to the user to convert them to lower resolution if desired).