In the upcoming change, device pixel conversion of ClipFrame will
happen during display list replay, where PaintContext is not available,
so let’s move it out of PaintContext.
The `as_corner()` and `floored_device_pixels()` functions popped up
frequently in profiles when selecting text on some Tweakers.net pages.
For every corner we're performing multiple device pixel calculations
regardless of whether any radius was actually set.
Add an early return if no radius is set. On my machine this reduces the
time taken in both `as_corner()` and `floored_device_pixels()` by 46%
(63% fewer calls).
