Add SkLights class

The SkLights class is intended to be passed to the drawLitAtlas call (and then on to SkLightingShader).

This CL is calved off of https://codereview.chromium.org/1261433009/ (Add SkCanvas::drawLitAtlas call).
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1255133004

Review-Url: https://codereview.chromium.org/1255133004
diff --git a/gyp/core.gypi b/gyp/core.gypi
index 2ae75be..dfe2432 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -369,6 +369,7 @@
         '<(skia_include_path)/core/SkImageEncoder.h',
         '<(skia_include_path)/core/SkImageFilter.h',
         '<(skia_include_path)/core/SkImageInfo.h',
+        '<(skia_include_path)/core/SkLights.h',
         '<(skia_include_path)/core/SkMallocPixelRef.h',
         '<(skia_include_path)/core/SkMask.h',
         '<(skia_include_path)/core/SkMaskFilter.h',