This will allow users to avoid including Painter.h
(cherry picked from commit 254d040ff4f81a0e04364d5d29a25c98d580cbb5;
amended to fix conflict in Painter.cpp because we didn't remove the
scale concept, and to keep a forwarding enum in place until we've
converted the rest of our code.)
Previously, calling `.right()` on a `Gfx::Rect` would return the last
column's coordinate still inside the rectangle, or `left + width - 1`.
This is called 'endpoint inclusive' and does not make a lot of sense for
`Gfx::Rect<float>` where a rectangle of width 5 at position (0, 0) would
return 4 as its right side. This same problem exists for `.bottom()`.
This changes `Gfx::Rect` to be endpoint exclusive, which gives us the
nice property that `width = right - left` and `height = bottom - top`.
It enables us to treat `Gfx::Rect<int>` and `Gfx::Rect<float>` exactly
the same.
All users of `Gfx::Rect` have been updated accordingly.
