Add constraint to prevent more than one normal map per drawable

[xeolabs]

For each drawable, only use the normal map texture node that's closest to the geometry on the parent node path.

Reasons

• A single pre-generated tangent array is given to the vertex shader, for the UV layer that's used for morphing.
• The vertex stage generates the tangent-bitangent-normal matrix.
• The vertex state multiplies the View-space eye vector by that matrix, then passes the tangent-space eye vector to the fragment stage as a varying.

Therefore, were we to support multiple normal maps, we would need a varying for an eye vector for each of those normal maps, which we can't afford.

There are no use cases for multiple normal maps anyway.