Improve biome blending in 3D

[aTom3333]

This PR improves performance of doing biome blending during chunk loading. The combined time of step 3 and 4 (loading chunks and finalization ; biome blending occurs during step 4) has roughly been halved based on my rough timings. (Sorry, no precise timings provided, I've been winging it a bit). The improved performance and the improved asymptotic complexity of blur make it reasonable to do blur bigger than 333 so that's now possible and exposed in the UI. As an added bonus that wasn't really intentional, memory usage has decreased when using trivial 3D biome structure (by virtue of not storing biome color for every block, more on that later).

The improved performances are achieved for 3 reasons:

• When possible a 2D blur is used instead of a 3D one. To know when using a 2D blur can be used (will give the same result as the 3D one), I keep track of vertical biome transition during chunk loading, ie when the biome at a block is different than the biome at the block just under. When a transition occurs, for every y level that is close to transition (closer than the blur radius), a 3D blur is needed as the biomes are not vertically uniform. Basically, for each y at which a (vertical) biome transition occurs, there is an interval of y value for which a 3D blur must be used. But for every y level outside of all of those intervals, a 2D blur is enough. Even better, there is usually a lot of consecutive y levels with (vertically) uniform biomes, this means the 2D blur can be computed for only one of those levels and copied for the other levels. In practice I don't track biome transition for each (x, z) column but for each chunk, this means that if there is a biome transition in a single column of the chunk, the 3D blur will be used for the entire chunk (for the y levels around the transition). And even more, because neighboring chunks affect biome blending, a single biome transition in a column will affect the whole chunk of the block and the neighboring chunks. To be effective this optimization relies on the fact that there isn't a lot of vertical biome transition in a minecraft world (which seems to be the case in my very limited testing). (this optimization does nothing to help performance when using 2D biome only)
• The blur computations, both 2D and 3D, have been changed to use a summed-area table to reduce the complexity, especially with bigger blur radius. (while writing this message I realized that in asymptotic sense, it doesn't actually improve the complexity the way I've done it. It's better then naive blur but maybe an separated blur is even better but that would be a separated PR). This optimization benefits the 2D biome only case as well.
• Only compute the biome colors when needed. That's a very rough way to do it (it could be done more finaly later I guess), during loading chunk I keep track of the min and max y value for which at least one block of the chunk needs biome color. When comes times to compute biome color, I don't compute any color outside of this interval. This simple test can lead to big wins on some chunks (like when there is no water underground, the biome colors will only be computed around surface level). (doesn't help in the 2D biome case). This is the change that cause the memory usage reduction, because in some cases a lot less biomes colors are stored.

The setting for enabling/disabling biome blending has been replace for a setting to choose the biome blending radius (and a radius of 0 means biome blending is disabled)

Sorry no screenshot of bigger radius, I'm too lazy and my test scene is stupid looking anyway

[aTom3333]

The third optimization is smart… Could that save memory in 2d biomes too because we could have a "don't care" biome then? Similar how we have any nodes in the octree?

It could be applicable with limited changes to the 2D case as well, if no block in the chunk uses biome color, the color for the biome doesn't need to be stored. We don't even need a "don't care" thing, we just won't store the color in the world texture in the first place.

[leMaik]

Something strange is happening with the biome colors…

[leMaik]

Rebased and resolved conflicts, I'll test again and merge this if it works now. :+1: