Fix mouse cacluations to account for non-integer
canvas scale, and rect posistion & size, and
replace Math.floor() with Math.round(). Fixes
- Unreachable max x/y value in some cases.
- Disproportionate number of mouse pixels
   for min and max mouse positions.

Ignore canvas borders and padding. This has
the nice benefit that it's much easier to land
on min/max mouse x/y, especially when moving
the mouse quickly.

Values remain centered until mouse move events are received.