A simple hough transformation application written in C

Status April 13th:
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Find centre of a black and white image correctly in a simple threaded way

Problems:
* At the moment threads are created in a way so that each thread will handle a certain number of rows and then quit. No
  new threads are created when threads finish, which means that often all but one thread will quit leaving one or more
  cores unused on mulitcore processors
* Memory usage is quite high
* Sobel operator gives a two pixel edge of white area which doubles the number of nescessay calculations

Optimizations to be done:
* Spawn a new thread when another thread quits. This will mean full usage of all cores during the whole calculation.
* Use SIMD of some sort to calculate the square root in thread_hough
* Implement thinning to make edges one pixel

Other things:
* Color filtering of color images would be a nice addition(easy to implement)
* Refactor code. A lot of globals are used which causes all code to be in one file.