Ensure that we never round the canvas dimensions above `maxCanvasPixels`
by rounding them to the preceeding multiple of the display ratio rather
than the succeeding one.
This commit introduces a test to ensure that:
- When zooming below the maxCanvasPixels limit, the canvas is rendered
with the correct size (the two sides are multiplied by the zoom factor).
- When zooming above the maxCanvasPixels limit, the canvas size is capped.
This commit adds a test for 0603d1ac18.
Before the fix the `pagerendered` events would be fired just 2-3
milliseconds after the call to `increaseScale`/`decreaseScale`.
