Efficient undo history

[1j01]

JS Paint allows way more undos than old versions of MS Paint (3 in Win95/98, and 50 in Win7), but it's way less efficient about it. It currently just naively stores the entire image for every undo state.

It should use rectangle-bounded operations. This is partially implemented in Mopaint.

[1j01]

To fix "out of memory" errors:

• handle "out of memory" errors explicitly at places where ImageData objects are created
  • make_or_update_undoable
  • canvas_has_any_apparent_image_data (this is part of code meant to detect an out-of-memory scenario, but I didn't realize it can also fail due to lack of memory!)
  • etc.
• make undo history more efficient
  • instead of storing the image data for the entire document in every undo state, store just a patch of the region that changed
  • (MS Paint does this, and I already do this in Textual Paint and Mopaint)
  • compress undo state patches using run-length encoding (RLE) or an image format like PNG
    • (MS Paint uses RLE)
  • and/or, store the actions done to the document, e.g. "line from (x1,x1) to (x2,y2)" rather than the resulting pixels
  • (I do this in Mopaint, to allow for editing operations after the fact.)
• store undo history on disk
  • (this way, even if you have to reload the page, you still have your undo history, and it should allow more undo history to be stored)
  • (I already do this in Mopaint, and it's very nice!)
  • still want to cache the undo history in memory, for speed, and handle the case where persistent storage is disallowed in the browser settings