Blender Git Loki

* disabled openmp for now
* created a function calc_area_normal_and_flatten_center to get rid of the loop over head of calculating them seperately
* changed stroke spacing to be a percentage of brush size instead of absolute pixels
* UI for stroke spacing now goes from 1-500 percent
* the power of overlapping strokes is attenuated by one minus of the amount of circle overlap
* tweaked clay brush strength upwards to account for attenuation
* clay now obey's the accumulate checkbox