Materials: normal map `amount` is not applied

[bcamper]

trafficstars

The amount parameter for material normal maps is parsed and set as a uniform (normalAmount), but it is not actually applied in the shader (material.glsl).

We can either fix this (pretty easily), or remove the functionality, since it can also be easily accomplished through a normal shader block. If the latter, we need to explain in the docs that amount is a parameter that applies to all texture types except normal maps (which is also odd).

[bcamper]

@blair1618 This also appears to be the case in ES, though for good reason since the material.glsl is (near?) identical to the JS version.