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.

Tests/LibGfx/CMakeLists.txt

@@ -5,6 +5,7 @@ set(TEST_SOURCES
     TestFontHandling.cpp
     TestICCProfile.cpp
     TestImageDecoder.cpp
+    TestRect.cpp
     TestScalingFunctions.cpp
 )