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.

Test/hlsl.tx.overload.frag

@@ -0,0 +1,17 @@
+
+Texture2D<float>  tf1;
+Texture2D<float4> tf4;
+
+RWTexture2D<float>  twf1;
+RWTexture2D<float4> twf4;
+
+float Func(Texture2D<float> DummyTex) { return 1.0f; }
+float4 Func(Texture2D<float4> DummyTex) { return float4(0,0,0,0); }
+
+float Func(RWTexture2D<float> DummyTex) { return 1.0f; }
+float4 Func(RWTexture2D<float4> DummyTex) { return float4(0,0,0,0); }
+
+float4 main() : SV_TARGET
+{
+    return Func(tf1) + Func(tf4) + Func(twf1) + Func(twf4);
+}