The default canvas size is 300x150 pixels. If the element or document
we are trying to screenshot for the WebDriver is not at least that size,
then we will create a canvas that is wider or taller than the actual
element we are painting, resulting in a bunch of transparent pixels
falling off the end.
This fixes 14 WPT css/CSS2/floats tests that we run in CI, and
presumably a ton of other reftests in the WPT test suite.
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