Force-update shadows when the world is changed

[x2048]

This PR attempts to fix the "lagging shadows" problem when shadow_update_frames setting is greater than 1.

shadow_update_frames setting was introduced to mitigate the performance degradation caused by shadow mapping and spreads rendering of the 'map' shadow over the given number of frames. This delays change in the shadow when the map changes (mining or building) and causes artifacts such as nodes not casting shadows on building or shadows cast by air on mining.

This PR forces a full SM render on the frame where a map change occurs, removing the lag completely.

The downside is that shadow update frequency is not even, which may be noticeable with fast-moving shadows (e.g. shadows of high cliffs).

To do

This PR is a Ready for Review.

How to test

1. Set shadow_update_frames to 16
2. Start a world and start digging/building/pour water etc.
3. Notice that shadows are updated immediately

[lhofhansl]

Is that even a problem worth fixing?

[x2048]

@lhofhansl
The problem is quite visible with shadow_update_frames > 8, and I have received some feedback supporting this.

There is performance gain in using shadow_update_frame, and this patch removes the main annoyance it brings.

[lhofhansl]

I see. Makes sense.
I think we should also (in the other PR) then leave the shadow_update_frame at what it is now (8) and also allow 16.

[lhofhansl]

Works.

[sfan5]

LGTM