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/SystemMonitor/ProcessMemoryMapWidget.cpp

@@ -42,7 +42,7 @@ public:
             else
                 VERIFY_NOT_REACHED();
 
-            painter.draw_line({ x, rect.top() }, { x, rect.bottom() }, color);
+            painter.draw_line({ x, rect.top() }, { x, rect.bottom() - 1 }, color);
         }
 
         painter.draw_rect(rect, Color::Black);