Add sampleVariablesStencilSupport to sksl standalone caps

TBR=ethannicholas@google.com

Change-Id: I53c2896470fd202f01f51c5aaddf3087b2cbb5ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251556
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>

src/sksl/README

@@ -24,7 +24,7 @@ Differences from GLSL
   the same as if and switch in all respects other than it being a compile-time
   error to use a non-constant expression as a test.
 * GLSL caps can be referenced via the syntax 'sk_Caps.<name>', e.g.
-  sk_Caps.sampleVariablesSupport. The value will be a constant boolean or int,
+  sk_Caps.canUseAnyFunctionInShader. The value will be a constant boolean or int,
   as appropriate. As SkSL supports constant folding and branch elimination, this
   means that an 'if' statement which statically queries a cap will collapse down
   to the chosen branch, meaning that:

src/sksl/SkSLIRGenerator.cpp

@@ -127,7 +127,6 @@ static void fill_caps(const SKSL_CAPS_CLASS& caps,
     CAP(fbFetchNeedsCustomOutput);
     CAP(flatInterpolationSupport);
     CAP(noperspectiveInterpolationSupport);
-    CAP(sampleVariablesSupport);
     CAP(externalTextureSupport);
     CAP(mustEnableAdvBlendEqs);
     CAP(mustEnableSpecificAdvBlendEqs);

src/sksl/SkSLUtil.h

@@ -100,6 +100,10 @@ public:
         return true;
     }
 
+    bool sampleVariablesStencilSupport() const {
+        return true;
+    }
+
     bool externalTextureSupport() const {
         return true;
     }