Preserve dithering for SkImageFilters::Shader

Migrating chromium code to use SkImageFilters::Shader over
SkPaintImageFilter requires dithering to be preserved for its gradient
fills.

I debated always forcing it to true, but dithering was never turned on
for the turbulence filter and it's not necessary for const color shaders
Given that, I opted to just make it a parameter to the filter factory,
which seems okay since we're unlikely to embed dithering into SkShader
itself, it's a shading-related parameter of SkPaint, and if we migrate
to always dithering, then we can remove it.

Bug: skia:9310
Change-Id: I86f14969e2446f3a84e71e687cb263bcd44cf9d9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/338156
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>

RELEASE_NOTES.txt

@@ -6,6 +6,8 @@ This file includes a list of high level updates for each milestone release.
 
 Milestone 89
 ------------
+  * Added dither control to the SkImageFilters::Shader factory.
+    https://review.skia.org/338156
 
   * Add MTLBinaryArchive parameter to GrMtlBackendContext. This allows
     Skia to cache PipelineStates in the given archive for faster

include/effects/SkImageFilters.h

@@ -304,6 +304,11 @@ public:
         return Picture(std::move(pic), target);
     }
 
+    enum class Dither : bool {
+        kNo = false,
+        kYes = true
+    };
+
     /**
      *  Create a filter that fills the output with the per-pixel evaluation of the SkShader. The
      *  shader is defined in the image filter's local coordinate system, so will automatically
@@ -315,12 +320,17 @@ public:
      *  @param shader The shader that fills the result image
      */
     static sk_sp<SkImageFilter> Shader(sk_sp<SkShader> shader, const CropRect& cropRect = {}) {
-        return Shader(std::move(shader), kNone_SkFilterQuality, cropRect);
+        return Shader(std::move(shader), Dither::kNo, cropRect);
+    }
+    static sk_sp<SkImageFilter> Shader(sk_sp<SkShader> shader, Dither dither,
+                                       const CropRect& cropRect = {}) {
+        return Shader(std::move(shader), dither, kNone_SkFilterQuality, cropRect);
     }
     // As above, but the filter quality defines what is used in image shaders that defer to the
     // quality stored on an SkPaint. NOTE: this default behavior is deprecated, so prefer creating
-    // image shaders with explicit sampling parameters and call the 2-arg Shader() factory instead.
-    static sk_sp<SkImageFilter> Shader(sk_sp<SkShader> shader, SkFilterQuality filterQuality,
+    // image shaders with explicit sampling parameters and call the other Shader() factory instead.
+    static sk_sp<SkImageFilter> Shader(sk_sp<SkShader> shader, Dither dither,
+                                       SkFilterQuality filterQuality,
                                        const CropRect& cropRect = {});
 
     /**

src/effects/imagefilters/SkImageFilters.cpp

@@ -194,11 +194,13 @@ sk_sp<SkImageFilter> SkImageFilters::Picture(sk_sp<SkPicture> pic, const SkRect&
     return SkPictureImageFilter::Make(std::move(pic), targetRect);
 }
 
-sk_sp<SkImageFilter> SkImageFilters::Shader(sk_sp<SkShader> shader, SkFilterQuality filterQuality,
+sk_sp<SkImageFilter> SkImageFilters::Shader(sk_sp<SkShader> shader, Dither dither,
+                                            SkFilterQuality filterQuality,
                                             const CropRect& cropRect) {
     SkImageFilter::CropRect r = to_legacy_crop_rect(cropRect);
     SkPaint paint;
     paint.setShader(std::move(shader));
+    paint.setDither((bool) dither);
     // For SkImage::makeShader() shaders using SkImageShader::kInheritFromPaint sampling options
     paint.setFilterQuality(filterQuality);
     return SkPaintImageFilter::Make(paint, &r);