Investigate correctness of canvas DevicePixel and CssPixel container sizing

PR #2361 fixes an issue where we weren't properly checking if the canvas needed to be redrawn. The issue was fixed, but I didn't take enough time to figure out and test the correctness of the canvas sizing. The issue is that DevicePixels are integers, and style values are CSS. We need a proper way to size things so that the canvas doesn't overflow the container, and we only update the size when the canvas is resized. The current implementation can have a pixel gap on the edge currently.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-233)