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/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; }