GSoC 2020: fix Fill Layer preview and resizing

In !467 (merged), I added live previewing of Fill Layers. But, as it can be expected, I introduced two glitches:

• No updates when the layer is resized (!467 (comment 90578))
• Clone layers not being updated (!467 (comment 90588))

As I tested this, I discovered an additional glitch that existed before my GSoC work:

• Fill Layers incorrectly preserve rendered quadrants when resized. This results in glitches similar to Wolthera's

Test Plan

Follow Dmitry's comments linked above. Additionally:

1. Add a Fill Layer (use SeExpr or Wolthera's generator for better results).
2. Resize the canvas.
3. Check that the layer renders the pattern correctly.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.