Quantize light frustum calculations

[x2048]

Reduces dancing shadows.

Quantization is applied to three variables:

• Camera/player position is truncated to 2-node increments
• Player camera direction is truncated to increments of 15 degrees in Yaw and Pitch
• Light direction is truncated to increments of 0.0625 degrees (approximate angle that sun travels in 15 seconds)

While reducing dancing of the shadows on the grass and tree leaves, the movement of the shadows becomes chunky.

To do

This PR is Ready for Review.

How to test

1. Enable shadows
2. Disable filters or reduce filters
3. Start game
4. Walk and look around, observe less dancing shadows
5. Pause time (set time speed to 0)
6. Walk and look around, there should be almost no dancing shadows.

@lhofhansl @kilbith You use shadows frequently, you feedback is appreciated
@Andrey2470T It might resolve #11743

[lhofhansl]

Yeah. The jittering is reduced a lot!
Code looks good.