[svg] Add light source classes and lighting-color pres attr

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

The three classes represent light source elements that will eventually
be used for feSpecularLighting and feDiffuseLighting. Currently they are
unused.

Also added the (currently unused) lighting-color presentation attribute.

Bug: skia:10841
Change-Id: Ic7824671662b8cd88cf627affc54173d5e881b7d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/359557
Commit-Queue: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
diff --git a/modules/svg/src/SkSVGAttribute.cpp b/modules/svg/src/SkSVGAttribute.cpp
index 098dc7b..f636adb 100644
--- a/modules/svg/src/SkSVGAttribute.cpp
+++ b/modules/svg/src/SkSVGAttribute.cpp
@@ -40,6 +40,7 @@
     result.fStopOpacity.set(SkSVGNumberType(1));
     result.fFloodColor.set(SkSVGColor(SK_ColorBLACK));
     result.fFloodOpacity.set(SkSVGNumberType(1));
+    result.fLightingColor.set(SkSVGColor(SK_ColorWHITE));
 
     return result;
 }