A very common workflow for painting in 2d and 3d is to draw a quick lasso selection or poly selection to mask the strokes and then paint. Instamat offers great masking for paint layers, but these are more work to set up not an ephemeral masks, often one that is only used for a handful of strokes.
Justification
A typical use would be when painting a shadowy area on rocks (may not be literally shadowed, could be moistere or moss). You would want part of the area to have a hard falloff and part to have a smooth falloff. You could quickly lasso the screen across the rock and then airbrush in over that.
Implementation Details
With each paint stroke that is stored, store also a screen space polygon mask.
