LibGfx+Everywhere: Change Gfx::Rect to be endpoint exclusive

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.

Userland/Applications/GamesSettings/ChessSettingsWidget.cpp

@@ -167,7 +167,7 @@ private:
         auto rect_for_square = [&](Chess::Square const& square) {
             return Gfx::IntRect {
                 frame_inner_rect().left() + square.file * square_size,
-                frame_inner_rect().bottom() + 1 - (square.rank + 1) * square_size,
+                frame_inner_rect().bottom() - (square.rank + 1) * square_size,
                 square_size,
                 square_size
             };