Simplified for loop for some image manipulation functions

[Minnowo]

The image manipulation functions ImageColorInvert, ImageColorBrightness, ImageColorReplace, and ImageColorTint all used a nested for loop with some math to index the 1d array of pixels.

These loops could be simplified to a single loop with a counter. I think this is not only more readable, but it has also shown some slight speed increases for very large images. My numbers below using the diff of time.h's clock function for a 7096 x 10031 image:

before:

brightness 0.932933
replace 0.910925
tint 1.116619
invert 0.785766

after:

brightness 0.784316
replace 0.782897
tint 0.934186
invert 0.699755

[raysan5]

@Minnowo Nice improvement! Thank you very much!