LibWeb: Add initial implementation of CRC2D.globalAlpha

Works for fills and strokes (using colors, gradients, or patterns),
along with images.

fill_rect() has been updated to use fill_path(), which allows it to
easily transform the rect, and already supports opacity.

Co-authored-by: MacDue <macdue@dueutil.tech>

Userland/Libraries/LibWeb/HTML/Canvas/CanvasCompositing.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/HTML/ImageData.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/canvas.html#canvascompositing
+class CanvasCompositing {
+public:
+    virtual ~CanvasCompositing() = default;
+
+    virtual float global_alpha() const = 0;
+    virtual void set_global_alpha(float) = 0;
+
+protected:
+    CanvasCompositing() = default;
+};
+
+}