PtLayer should allow changing colours with Python

[dpogue]

I know we added it for a fairly specific use case, but it would be cool for a ptAttribLayer to allow Python to adjust the layer colours rather than just the image.

This could enable things like tintable Relto additions.

[Deledrius]

That sounds like a pretty useful addition.

[TheScarFr]

Oh! I currently work on something for which this would be very useful! :)