[Skia] Implement support for shadows that ignore transforms

https://bugs.webkit.org/show_bug.cgi?id=270175

Reviewed by Carlos Garcia Campos and Nikolas Zimmermann.

When creating the drop shadow filter, if state.shadowsIgnoreTransforms()
is true, render the drop shadow with trasformed coordinates and blur
radius, by adjusting them against the CTM inverse.

This is enough to make various canvas tests happy. Some of them are still
failing with off-by-one colors, and it's not clear why.

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

Canonical link: https://commits.webkit.org/275492@main

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

@@ -41,6 +41,7 @@
 #include <skia/core/SkPath.h>
 #include <skia/core/SkPathEffect.h>
 #include <skia/core/SkPathTypes.h>
+#include <skia/core/SkPoint3.h>
 #include <skia/core/SkRRect.h>
 #include <skia/core/SkRegion.h>
 #include <skia/core/SkTileMode.h>
@@ -345,25 +346,46 @@ void GraphicsContextSkia::strokePath(const Path& path)
 
 sk_sp<SkImageFilter> GraphicsContextSkia::createDropShadowFilterIfNeeded(ShadowStyle shadowStyle) const
 {
-    // FIXME: this does not handle state.shadowsIgnoreTransforms()
-
     if (!hasDropShadow())
         return nullptr;
 
     const auto& shadow = dropShadow();
     ASSERT(shadow);
 
-    const FloatSize& offset = shadow->offset;
+    auto offset = shadow->offset;
     const auto& shadowColor = shadow->color;
 
     if (!shadowColor.isVisible() || (!offset.width() && !offset.height() && !shadow->radius))
         return nullptr;
 
     const auto& state = this->state();
-    const auto sigma = shadow->radius / 2.0;
+    auto sigma = shadow->radius / 2.0;
 
     switch (shadowStyle) {
     case ShadowStyle::Outset:
+        if (state.shadowsIgnoreTransforms()) {
+            // When state.shadowsIgnoreTransforms() is true, the offset is in
+            // natural orientation for the Y axis, like CG. Convert that back to
+            // the Skia coordinate system.
+            offset.scale(1.0, -1.0);
+
+            // Fast path: identity CTM doesn't need the transform compensation
+            AffineTransform ctm = getCTM(GraphicsContext::IncludeDeviceScale::PossiblyIncludeDeviceScale);
+            if (ctm.isIdentity())
+                return SkImageFilters::DropShadow(offset.width(), offset.height(), sigma, sigma, shadowColor.colorWithAlphaMultipliedBy(state.alpha()), nullptr);
+
+            // Ignoring the CTM is practically equal as applying the inverse of
+            // the CTM when post-processing the drop shadow.
+            if (const std::optional<SkMatrix>& inverse = ctm.inverse()) {
+                SkPoint3 p = SkPoint3::Make(offset.width(), offset.height(), 0);
+                inverse->mapHomogeneousPoints(&p, &p, 1);
+                sigma = inverse->mapRadius(sigma);
+                return SkImageFilters::DropShadow(p.x(), p.y(), sigma, sigma, shadowColor.colorWithAlphaMultipliedBy(state.alpha()), nullptr);
+            }
+
+            return nullptr;
+        }
+
         return SkImageFilters::DropShadow(offset.width(), offset.height(), sigma, sigma, shadowColor.colorWithAlphaMultipliedBy(state.alpha()), nullptr);
     case ShadowStyle::Inset: {
         auto dropShadow = SkImageFilters::DropShadowOnly(offset.width(), offset.height(), sigma, sigma, SK_ColorBLACK, nullptr);