[svg] Implement feDiffuseLighting

https://www.w3.org/TR/SVG11/filters.html#feDiffuseLightingElement

- Add SkSVGFeDiffuseLighting node
- Move distant light source direction computation into a method on
  SkSVGFeDistantLight
- Implement distant and point light sources for feDiffuseLighting

Bug: skia:10841
Change-Id: I74b8b9e04be5d2c5ac9f912d015dce96367040a1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/402645
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
diff --git a/modules/svg/include/SkSVGFeLightSource.h b/modules/svg/include/SkSVGFeLightSource.h
index 75f0074..643c150 100644
--- a/modules/svg/include/SkSVGFeLightSource.h
+++ b/modules/svg/include/SkSVGFeLightSource.h
@@ -8,6 +8,7 @@
 #ifndef SkSVGFeLightSource_DEFINED
 #define SkSVGFeLightSource_DEFINED
 
+#include "include/core/SkPoint3.h"
 #include "modules/svg/include/SkSVGHiddenContainer.h"
 #include "modules/svg/include/SkSVGTypes.h"
 
@@ -30,6 +31,8 @@
         return sk_sp<SkSVGFeDistantLight>(new SkSVGFeDistantLight());
     }
 
+    SkPoint3 computeDirection() const;
+
     SVG_ATTR(Azimuth  , SkSVGNumberType, 0)
     SVG_ATTR(Elevation, SkSVGNumberType, 0)