This change makes RecordingPainter to emit a FillRect command instead
of FillRectWithRoundedCorners if all corners have a radius = 0.
`fill_rect_with_rounded_corners()` in LibGfx already has a similar
optimization. But now when we also have LibAccelGfx, which does not
support painting rectangles with rounded corners yet, it makes sense to
emit FillRect whenever possible.
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