Make CSSPixels::to_float(), to_double(), and to_int() constexpr inline
instead of out-of-line in the .cpp file. These are trivial one-liners
called in very hot painting paths.
Also optimize DevicePixelConverter::rounded_device_rect() to work
directly with CSSPixels values instead of constructing an intermediate
Rect<double> and scaling it.
When rounding a CSSPixelRect to a DevicePixelRect, we simply pulled its
width and height through round() and called it a day. Unfortunately this
could negatively affect the rect's perceived positioning.
A rect at { 0.5, 0.0 } with size { 19.5 x 20.0 } should have its right
edge at position 20, but after rounding it would end up at { 1, 0 } with
size { 20 x 20 }, causing its right edge to be at position 21 instead.
Fix this by first rounding the right and bottom edges of the input rect,
and then determining the dimensions by subtracting its rounded position.
Fixes#245.
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.
