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/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index e9fc865..d9d323d 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -434,7 +434,7 @@
                     ge, t.c_str(),
                     args.fOutputColor,
                     args.fInputColor,
-                    args.fSamplers);
+                    args.fTexSamplers);
 }
 
 /////////////////////////////////////////////////////////////////////