When saving thumbnails of a match, don't save the background paper image - just save the match's strokes with alpha

this should result in a smaller PNG, and it will let me switch paper textures in the future.
Fix rendering of eraser strokes in thumbnails

Manually multiply compositing the thumbnail image over the paper texture in a background render queue
Add drawing delete functionality to the gallery

- when long pressing a drawing, all drawings start wiggling, and show an (x)
- when the (x) is tapped, the drawing is deleted from the store and the collection view updates