Rename EmitArgs::fSamplers to fTexSamplers

Renames fSamplers to fTexSamplers in GrProcessor EmitArgs, and renames
GrGLSLTextureSampler to GrGLSLSampler. This will allow us to add a
second array of buffer samplers.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1862373003

Review URL: https://codereview.chromium.org/1862373003
diff --git a/src/gpu/effects/GrSimpleTextureEffect.cpp b/src/gpu/effects/GrSimpleTextureEffect.cpp
index 904b898..f4cbd06 100644
--- a/src/gpu/effects/GrSimpleTextureEffect.cpp
+++ b/src/gpu/effects/GrSimpleTextureEffect.cpp
@@ -17,7 +17,7 @@
         GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
         fragBuilder->codeAppendf("%s = ", args.fOutputColor);
         fragBuilder->appendTextureLookupAndModulate(args.fInputColor,
-                                                  args.fSamplers[0],
+                                                  args.fTexSamplers[0],
                                                   args.fCoords[0].c_str(),
                                                   args.fCoords[0].getType());
         fragBuilder->codeAppend(";");