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/TestRect.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Jelle Raaijmakers <jelle@gmta.nl>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/Rect.h>
+#include <LibTest/TestCase.h>
+
+TEST_CASE(int_rect_right_and_bottom)
+{
+    Gfx::IntRect rect = { 2, 3, 4, 5 };
+    EXPECT_EQ(rect.right(), 6);
+    EXPECT_EQ(rect.bottom(), 8);
+}
+
+TEST_CASE(float_rect_right_and_bottom)
+{
+    Gfx::FloatRect rect = { 1.f, 2.f, 3.5f, 4.5f };
+    EXPECT_APPROXIMATE(rect.right(), 4.5f);
+    EXPECT_APPROXIMATE(rect.bottom(), 6.5f);
+}
+
+TEST_CASE(rect_contains_vertically)
+{
+    Gfx::FloatRect rect = { 0.f, 0.f, 100.f, 100.f };
+    EXPECT(rect.contains_vertically(99.f));
+    EXPECT(!rect.contains_vertically(100.f));
+}
+
+TEST_CASE(rect_shatter)
+{
+    Gfx::IntRect glass_plate = { 0, 0, 100, 100 };
+    Gfx::IntRect hammer = { 30, 40, 40, 10 };
+
+    auto shards = glass_plate.shatter(hammer);
+    EXPECT(!shards.is_empty());
+
+    int total_shard_area = 0;
+    for (auto shard : shards) {
+        EXPECT(glass_plate.contains(shard));
+        EXPECT(!hammer.intersects(shard));
+        total_shard_area += shard.size().area();
+    }
+
+    EXPECT_EQ(glass_plate.size().area() - hammer.size().area(), total_shard_area);
+}