Suggestion - Joy of Painting: Make canvases stackable

[hecko-yes]

The suggestion is kind of twofold:

1. Make blank canvases stackable

   • When attempting to paint on a stack of canvases, it'd probably make the most sense to keep one in the held slot and put the rest elsewhere (or drop if no room), opposite from vanilla bottles

2. Make (signed) paintings stackable

   • Vanilla written books already are, so surely it's possible
   • Might encourage spamming paintings, which might not be great for performance?