HLSL: add .mips[][] operator for texture types

This implements mytex.mips[mip][coord] for texture types.  There is
some error testing, but not comprehensive.  The constructs can be
nested, e.g in this case the inner .mips is parsed before the completion
of the outer [][] operator.

   tx.mips[tx.mips[a][b].x][c]

hlsl/hlslParseHelper.h

@@ -429,6 +429,17 @@ protected:
     TVector<TVariable*> implicitThisStack;   // currently active 'this' variables for nested structures
 
     TVariable* gsStreamOutput;               // geometry shader stream outputs, for emit (Append method)
+
+    // This tracks the first (mip level) argument to the .mips[][] operator.  Since this can be nested as
+    // in tx.mips[tx.mips[0][1].x][2], we need a stack.  We also track the TSourceLoc for error reporting 
+    // purposes.
+    struct tMipsOperatorData {
+        tMipsOperatorData(TSourceLoc l, TIntermTyped* m) : loc(l), mipLevel(m) { }
+        TSourceLoc loc;
+        TIntermTyped* mipLevel;
+    };
+
+    TVector<tMipsOperatorData> mipsOperatorMipArg;
 };
 
 // This is the prefix we use for builtin methods to avoid namespace collisions with