v1.3.0: Fix bugs to do with previews and null layers, optimizations (speedup)

It turns out GLM's matrix multiplication can be quite slow, so repeated
calculations are now done element-wise.
In addition, the rotation matrices for the viewpoint have been removed
in favor of a pre-optimized vector calculation.

This makes the plugin twice as fast with anti-aliasing on, and around 5x
as fast with it off.