Fixed a bug with linear gradient PDF matrices and added test cases

gyp/gmslides.gypi

 # include this gypi to include all the golden master slides.
 {
   'sources': [
     '../gm/aaclip.cpp',
     '../gm/aarectmodes.cpp',
     '../gm/alphagradients.cpp',
     '../gm/arithmode.cpp',
     '../gm/bicubicfilter.cpp',
     '../gm/bigmatrix.cpp',
     '../gm/bitmapcopy.cpp',
@@ skipping 29 matching lines @@
     '../gm/factory.cpp',
     '../gm/filltypes.cpp',
     '../gm/filltypespersp.cpp',
     '../gm/filterbitmap.cpp',
     '../gm/fontmgr.cpp',
     '../gm/fontscaler.cpp',
     '../gm/gammatext.cpp',
     '../gm/getpostextpath.cpp',
     '../gm/giantbitmap.cpp',
     '../gm/gradients.cpp',
+    '../gm/gradient_matrix.cpp',
     '../gm/gradtext.cpp',
     '../gm/hairlines.cpp',
     '../gm/hairmodes.cpp',
     '../gm/hittestpath.cpp',
     '../gm/imageblur.cpp',
     '../gm/imagemagnifier.cpp',
     '../gm/lerpmode.cpp',
     '../gm/lighting.cpp',
     '../gm/image.cpp',
     '../gm/imagefiltersbase.cpp',
@@ skipping 55 matching lines @@
     '../gm/xfermodes.cpp',
     '../gm/xfermodes2.cpp',
   ],
 }
 
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