Full stack: distinguish between a scalar and a vector of size 1.

There have been GLSL extensions considering this, and HLSL does it.
This is a fully backward compatible change that allows this distinction.

Test/hlsl.float1.frag

@@ -0,0 +1,7 @@
+float1 f1 = float1(1.0);
+float scalar = 2.0;
+
+float1 ShaderFunction(float1 inFloat1, float inScalar) : COLOR0
+{
+    return f1 * scalar + inFloat1 * inScalar;
+}