More brush settings

[don-tnowe]

More settings for brushes.

Some will need the drawing algo to change from set_pixel() drawing to objects-in-viewport rendering.

• [ ] Currently, brush strokes look weird if you zig-zag a soft brush. This is a side effect of an algorithm that gives more pen pressure opacity control - allow using the good-ol' "just draw a bunch of circles" algorithm.
• [ ] Paint with a texture instead of circles
• [ ] Blend modes on brushes
• [ ] Preserve underlying alpha
• [ ] Change cursor color

Note: when too many get added, they should be hidden somehow to not clutter workspace. Could be done through a toggle added via start_property_toggle(label), shown when pressed.

[DearFox]

Add minimum brush size when using graphics tablet input