For octahedral compressed normals/tangents, we use vec4 in the shader
regardless of whether a normal/tangent does/doesn't exist
For the case where we only have a normal vector, we need to specify that
there are only two components being used when calling glVertexAttrib
Before we would always specify that there were 4 components, and used
offsets to determine where in the vertex buffer to read data from but
this doesn't work on all platforms
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