Creating an image.Rectangle from a f32.Rectangle is used by two packages in Gio
and about to be used for a third. Add a Round method to f32.Rectangle to avoid
duplicating the implementation.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
layout.FRect, layout.FPt for converting from integer to floating point,
useful for drawing operations.
f32.Pt is a shorthand that mirrors image.Pt.
Signed-off-by: Elias Naur <mail@eliasnaur.com>