HLSL: allow name mangling based on texture template type

Name mangling did not account for the vector size in the template type of a texture.
This adds that.  The mangle is as it ever was for the vec4 case, which leaves
all GLSL behavior and most HLSL behavior uneffected.  For vec1-3 the size is added
to the mangle.

Current limitation: textures cannot presently be templatized on structured types,
so this works only for vectors of basic types.

Fixes #895.

gtests/Hlsl.FromFile.cpp

@@ -287,6 +287,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.swizzle.frag", "PixelShaderFunction"},
         {"hlsl.templatetypes.frag", "PixelShaderFunction"},
         {"hlsl.tx.bracket.frag", "main"},
+        {"hlsl.tx.overload.frag", "main"},
         {"hlsl.type.half.frag", "main"},
         {"hlsl.type.identifier.frag", "main"},
         {"hlsl.typeGraphCopy.vert", "main"},