Deterministic rain

Great effort has been made that multithreading doesn't sacrifice determinism. If you'd like to generate large images by splitting them into tiles, now you can. You just need to supply the same seed to all tiles, and they should join cleanly.

Changelog:

• Bug fix: Solved various SegFaults
• Bug fix: Corrupted files are now deleted and re-created
• Bug fix: Bands of iteration depths now work correctly
• Enhancement: Added optimiser for Mandelbrot Set to speed up search
• Feature: Added Burning Ship formula, and Julia set with hard-coded values
• Breaking change: The --runs parameter now specifies number of all attempted points, not just successful ones

First rain

Now the iterator works and everything, so here goes the first pre-release.