Precompute projection matrices when rendering 2D shadows

[clayjohn]

Small optimization I found while profiling CPU performance. light_update_shadow() was calculating the same matrices every time it was called even though they never change.

I also pulled the calculation of the projection matrix out of the loop (although I am 99% certain this was optimized out of the loop anyway in release builds).

Profiled when running the Isometric demo

Before: light_update_shadow() takes 17.1% of the CPU frame time

After: light_update_shadow() takes 14.7% of the CPU frame time. Notice that the from_euler chunk is gone completely

This demo is GPU-bound so this doesn't change the framerate.

[Calinou]

Tested locally, it works as expected. Basis::from_euler() is no longer present in profiles when running the 2D isometric demo and standing still for 300 frames.

[Repiteo]

Thanks!