Fix clipping not being applied to the source in drawCanvas() 9-argument version

[mpaperno]

The drawCanvas() test below should look like the drawImage() one above it. Same source image, same coordinates used, but source image first drawn another canvas at full size.

// tests['drawImage ....
  img.onload = function () {
    ctx.drawImage(img, 13, 13, 45, 45, 25, 25, img.width / 2, img.height / 2)
  }

// tests['drawCanvas ....
  img.onload = function () {
    const ctx2 = new Canvas(200,200).getContext('2d');
    ctx2.drawImage(img, 0, 0);
    ctx.drawCanvas(ctx2.canvas, 13, 13, 45, 45, 25, 25, img.width / 2, img.height / 2)
  }

The canvas' Picture can't be clipped, as far as I can tell, and there's no SkCanvs.draw_picture() version that takes a clip, like with draw_image_rect_..(), so I clipped the destination canvas instead.

This came up while I was working on SVG rendering and was doing a final check, expecting the current Content2D.draw_picture() to take care of things once given a Picture... then was surprised. Luckily thought to test with drawCanvas() first before assuming it had anything to do with SVG specifically.