Allow to change foreground color of Bitmap image

[birbilis]

Version: 0.7.1

should be able to change foreground color for a bitmap image (replace all non-transparent pixels with that)

note that compared to the same action that SVG supports, this would be a non-revertable action even if one clears the foreground color (currently UI doesn't allow to clear it back to default/clNull), since it would be too costly to keep two bitmaps in memory (to store the original one)

alternative is to do the operation via a shader instead of touching the bitmap (to set its opaque color)