Multithreading support

[roman901]

Very simple, but working region rendering parallelism, driven by ctpl library. Renderer took number of cores by std::threads::hardware_concurrency() and creates thread pool.

Speed of big worlds rendering dramatically increased

[roman901]

What do you think about this thing? (Bump)

[neocturne]

I'd like to have a command line argument for the number of threads, defaulting to 1. I'm running Minecraft and MinedMap on the same hardware, and MinedMap usually

• only regenerates very few tiles, making updates finish quickly -> multi-threading is not that useful
• has low priority, so using more than one core is undesirable

The multi-threading feature is very useful for the initial run or full regeneration of the map after MinedMap updates though, so I'm generally in favor of merging this.

[neocturne]

Also, one actual issue: the thread pool must be finished before makeMipmaps is called, as makeMipmaps uses the generated images as input.

In a second step, multi-threading should be added for makeMipmaps as well.

[roman901]

I'm use MinedMap on 30-40gb map and update took around a minute, multithreading makes in much faster.

OK, will see what I can do with mipmaps!