LibWeb: Add simple canvas path clipper

This adds CanvasPathClipper and ScopedCanvasPathClip. These allow
clipping the canvas by some arbitrary path.

This initial implementation is fairly naive, with a good few
allocations, though this can probably be improved in future.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -206,6 +206,7 @@ set(SOURCES
     HTML/BrowsingContextGroup.cpp
     HTML/Canvas/CanvasDrawImage.cpp
     HTML/Canvas/CanvasPath.cpp
+    HTML/Canvas/CanvasPathClipper.cpp
     HTML/Canvas/CanvasState.cpp
     HTML/CanvasGradient.cpp
     HTML/CanvasPattern.cpp

Userland/Libraries/LibWeb/HTML/Canvas/CanvasPathClipper.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/AntiAliasingPainter.h>
+#include <LibWeb/HTML/Canvas/CanvasPathClipper.h>
+
+namespace Web::HTML {
+
+// FIXME: This pretty naive, we should be able to cut down the allocations here
+// (especially for the paint style which is a bit sad).
+
+ErrorOr<CanvasPathClipper> CanvasPathClipper::create(Gfx::Painter& painter, CanvasClip const& canvas_clip)
+{
+    auto bounding_box = enclosing_int_rect(canvas_clip.path.bounding_box());
+    Gfx::IntRect actual_save_rect {};
+    auto maybe_bitmap = painter.get_region_bitmap(bounding_box, Gfx::BitmapFormat::BGRA8888, actual_save_rect);
+    RefPtr<Gfx::Bitmap> saved_clip_region;
+    if (!maybe_bitmap.is_error()) {
+        saved_clip_region = maybe_bitmap.release_value();
+    } else if (actual_save_rect.is_empty()) {
+        // This is okay, no need to report an error.
+    } else {
+        return maybe_bitmap.release_error();
+    }
+    painter.save();
+    painter.add_clip_rect(bounding_box);
+    return CanvasPathClipper(move(saved_clip_region), bounding_box, canvas_clip);
+}
+
+ErrorOr<void> CanvasPathClipper::apply_clip(Gfx::Painter& painter)
+{
+    painter.restore();
+    if (!m_saved_clip_region)
+        return {};
+    Gfx::IntRect actual_save_rect {};
+    auto clip_area = TRY(painter.get_region_bitmap(m_bounding_box, Gfx::BitmapFormat::BGRA8888, actual_save_rect));
+    painter.blit(actual_save_rect.location(), *m_saved_clip_region, m_saved_clip_region->rect(), 1.0f, false);
+    Gfx::AntiAliasingPainter aa_painter { painter };
+    auto fill_offset = m_bounding_box.location() - actual_save_rect.location();
+    aa_painter.fill_path(m_canvas_clip.path, TRY(Gfx::BitmapPaintStyle::create(clip_area, fill_offset)), m_canvas_clip.winding_rule);
+    return {};
+}
+}

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

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/Painter.h>
+#include <LibGfx/Path.h>
+
+namespace Web::HTML {
+
+struct CanvasClip {
+    Gfx::Path path;
+    Gfx::Painter::WindingRule winding_rule;
+};
+
+class CanvasPathClipper {
+public:
+    static ErrorOr<CanvasPathClipper> create(Gfx::Painter&, CanvasClip const& canvas_clip);
+    ErrorOr<void> apply_clip(Gfx::Painter& painter);
+
+private:
+    CanvasPathClipper(RefPtr<Gfx::Bitmap const> saved_clip_region, Gfx::IntRect bounding_box, CanvasClip const& canvas_clip)
+        : m_saved_clip_region(saved_clip_region)
+        , m_bounding_box(bounding_box)
+        , m_canvas_clip(canvas_clip)
+    {
+    }
+
+    RefPtr<Gfx::Bitmap const> m_saved_clip_region;
+    Gfx::IntRect m_bounding_box;
+    CanvasClip const& m_canvas_clip;
+};
+
+class ScopedCanvasPathClip {
+    AK_MAKE_NONMOVABLE(ScopedCanvasPathClip);
+    AK_MAKE_NONCOPYABLE(ScopedCanvasPathClip);
+
+public:
+    ScopedCanvasPathClip(Gfx::Painter& painter, Optional<CanvasClip> const& canvas_clip)
+        : m_painter(painter)
+    {
+        if (canvas_clip.has_value()) {
+            auto clipper = CanvasPathClipper::create(painter, *canvas_clip);
+            if (!clipper.is_error())
+                m_canvas_clipper = clipper.release_value();
+            else
+                dbgln("CRC2D Error: Failed to apply canvas clip path: {}", clipper.error());
+        }
+    }
+
+    ~ScopedCanvasPathClip()
+    {
+        if (m_canvas_clipper.has_value())
+            m_canvas_clipper->apply_clip(m_painter).release_value_but_fixme_should_propagate_errors();
+    }
+
+private:
+    Gfx::Painter& m_painter;
+    Optional<CanvasPathClipper> m_canvas_clipper;
+};
+
+}