When rounding a CSSPixelRect to a DevicePixelRect, we simply pulled its
width and height through round() and called it a day. Unfortunately this
could negatively affect the rect's perceived positioning.
A rect at { 0.5, 0.0 } with size { 19.5 x 20.0 } should have its right
edge at position 20, but after rounding it would end up at { 1, 0 } with
size { 20 x 20 }, causing its right edge to be at position 21 instead.
Fix this by first rounding the right and bottom edges of the input rect,
and then determining the dimensions by subtracting its rounded position.
Fixes#245.
With the newly supported fuzzy matching in our test-web runner, we can
now define the expected maximum color channel and pixel count errors per
failing test and set a baseline they should not exceed.
The figures I added to these tests all come from my macOS M4 machine.
Most discrepancies seem to come from color calculations being slightly
off.