[Skia] Implement outset drop shadows

https://bugs.webkit.org/show_bug.cgi?id=269596

Reviewed by Nikolas Zimmermann.

Whenever the GraphicsContext has a drop shadow set, use that to create
and set a SkImageFilters::DropShadow() image effect to the fill paint.
Stroke paints are unaffected.

The SkImageFilters::DropShadow() receives a blur sigma instead of radius
and the formal definition of that [1] states that sigma is half of the
blur radius, so convert between these values.

This only implements outset shadows. Inset shadows won't be rendered
anyway because GraphicsContextSkia::fillRectWithRoundedHole() is not
implemented yet.

[1] https://html.spec.whatwg.org/multipage/canvas.html#shadows

* Source/WebCore/platform/graphics/skia/GraphicsContextSkia.cpp:
(WebCore::GraphicsContextSkia::createFillPaint const):

Canonical link: https://commits.webkit.org/274928@main

Source/WebCore/platform/graphics/skia/GraphicsContextSkia.cpp

@@ -42,6 +42,7 @@
 #include <skia/core/SkRegion.h>
 #include <skia/core/SkTileMode.h>
 #include <skia/effects/SkDashPathEffect.h>
+#include <skia/effects/SkImageFilters.h>
 #include <wtf/MathExtras.h>
 
 #if USE(THEME_ADWAITA)
@@ -306,6 +307,21 @@ SkPaint GraphicsContextSkia::createFillPaint(std::optional<Color> fillColor) con
         paint.setColor(SkColorSetARGB(a, r, g, b));
     }
 
+    // Outset shadow
+    // FIXME: Don't add the effect if the shadow is inset
+    if (hasDropShadow()) {
+        const auto shadow = dropShadow();
+        ASSERT(shadow);
+
+        const auto sigma = shadow->radius / 2.0;
+        auto globalAlpha = state.alpha();
+        auto shadowColor = shadow->color;
+        if (globalAlpha < 1)
+            shadowColor = shadowColor.colorWithAlphaMultipliedBy(globalAlpha);
+        auto [r, g, b, a] = shadowColor.toColorTypeLossy<SRGBA<uint8_t>>().resolved();
+        paint.setImageFilter(SkImageFilters::DropShadow(shadow->offset.width(), shadow->offset.height(), sigma, sigma, SkColorSetARGB(a, r, g, b), nullptr));
+    }
+
     return paint;
 }