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;
+};
+
+}

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

@@ -0,0 +1,6 @@
+// https://html.spec.whatwg.org/multipage/canvas.html#canvascompositing
+interface mixin CanvasCompositing {
+  // compositing
+  attribute unrestricted double globalAlpha; // (default 1.0)
+// FIXME:  attribute DOMString globalCompositeOperation; // (default "source-over")
+};

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

@@ -78,6 +78,7 @@ public:
         float line_width { 1 };
         bool image_smoothing_enabled { true };
         Bindings::ImageSmoothingQuality image_smoothing_quality { Bindings::ImageSmoothingQuality::Low };
+        float global_alpha = { 1 };
         Optional<CanvasClip> clip;
     };
     DrawingState& drawing_state() { return m_drawing_state; }