Make color, fill, stroke et al. thread-safe: #3077

[GKFX]

#4774 improved. color should be entirely thread-safe; it should now also be impossible for fill and stroke (or background or whatever) to pass values between each other; i.e. fill(#009700) and stroke(#000056) running at once shouldn't lead to a fill color of 0xff000056, which was previously possible. In theory I think that running two fill() commands with different values at once might lead to the fillR etc. values being made from a mixture of the two colors, but I haven't been able to demonstrate this and I don't want to add any performance hit by making all fill methods synchronize on fillArray since using fill() in two threads is a highly unlikely situation.
Fixes #3077.