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.
