HLSL: fix preprocessor concatenation behaviour.

Fix #772.

Test/hlsl.preprocessor.frag

@@ -0,0 +1,13 @@
+#define DEFINE_TEXTURE(name) Texture2D name; SamplerState name##_ss;
+#define SAMPLE_TEXTURE(name, uv) name.Sample(name##_ss, (uv).xy)
+
+#define test_texture2 test_texture
+
+DEFINE_TEXTURE(test_texture)
+
+float4 main(float4 input : TEXCOORD0) : SV_TARGET
+{
+    float4 tex = SAMPLE_TEXTURE(test_texture2, input.xy);
+    return tex;
+}
+