Refactor how we store and use samplers in Ganesh

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 0b65188e83a52703974217045bfbf425d6c9b94e..0bff0fe49ec8e2ccdee4b300067b1af727361b6e 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -181,10 +181,11 @@ void GrGLAlphaThresholdEffect::emitCode(EmitArgs& args) {
     fragBuilder->codeAppendf("\t\tvec2 coord = %s;\n", coords2D.c_str());
     fragBuilder->codeAppendf("\t\tvec2 mask_coord = %s;\n", maskCoords2D.c_str());
     fragBuilder->codeAppend("\t\tvec4 input_color = ");
-    fragBuilder->appendTextureLookup(args.fTexSamplers[0], "coord");
+    fragBuilder->appendTextureLookup(uniformHandler->getSampler(args.fTexSamplers[0]), "coord");
     fragBuilder->codeAppend(";\n");
     fragBuilder->codeAppend("\t\tvec4 mask_color = ");
-    fragBuilder->appendTextureLookup(args.fTexSamplers[1], "mask_coord");
+    fragBuilder->appendTextureLookup(uniformHandler->getSampler(args.fTexSamplers[1]),
+                                     "mask_coord");
     fragBuilder->codeAppend(";\n");
 
     fragBuilder->codeAppendf("\t\tfloat inner_thresh = %s;\n",