Fixed bug when using multiple spot lights

<h3>Issue

As reported by @jwoolston in the revived thread of https://github.com/MasDennis/Rajawali/issues/227, shaders will fail to compile when using multiple spot lights. This was caused by `spot_factor` lacking an index within the light loops.

<h3> Resolution

This change appends an index to `spot_factor` in each of the base materials so that multiple spot lights can now be used. It does not impact `RajawaliExamples` since it does not utilize this light type.