Stop converting between CSS and device pixels as part of rendering - the
display list should be as simple as possible, so convert to DevicePixels
once when constructing the display list.
This change adds border-radius awareness to hit testing in two places:
1. ClipData::contains() now uses BorderRadiiData::contains() to properly
check if a point is inside a rounded clip rect. This handles overflow
clips from ancestor elements that have border-radius.
2. PaintableBox::hit_test() now directly checks the element's own
border-radius before reporting a hit.
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).
