Add convex polygon rendering effect and GM to test it.

gyp/gmslides.gypi

 # include this gypi to include all the golden master slides.
 {
   'sources': [
     '../gm/aaclip.cpp',
     '../gm/aarectmodes.cpp',
     '../gm/alphagradients.cpp',
     '../gm/androidfallback.cpp',
     '../gm/arcofzorro.cpp',
     '../gm/arithmode.cpp',
     '../gm/beziereffects.cpp',
@@ skipping 21 matching lines @@
     '../gm/clippedbitmapshaders.cpp',
     '../gm/coloremoji.cpp',
     '../gm/colorfilterimagefilter.cpp',
     '../gm/colormatrix.cpp',
     '../gm/colortype.cpp',
     '../gm/complexclip.cpp',
     '../gm/complexclip2.cpp',
     '../gm/composeshader.cpp',
     #'../gm/conicpaths.cpp',
     '../gm/convexpaths.cpp',
+    '../gm/convexpolyeffect.cpp',
     '../gm/copyTo4444.cpp',
     '../gm/cubicpaths.cpp',
     '../gm/cmykjpeg.cpp',
     '../gm/degeneratesegments.cpp',
     '../gm/dashcubics.cpp',
     '../gm/dashing.cpp',
     '../gm/deviceproperties.cpp',
     '../gm/distantclip.cpp',
     '../gm/displacement.cpp',
     '../gm/downsamplebitmap.cpp',
@@ skipping 97 matching lines @@
     '../gm/vertices.cpp',
     '../gm/verttext.cpp',
     '../gm/verttext2.cpp',
     '../gm/verylargebitmap.cpp',
     '../gm/xfermodeimagefilter.cpp',
     '../gm/xfermodes.cpp',
     '../gm/xfermodes2.cpp',
     '../gm/xfermodes3.cpp',
   ],
 }