Smal improve of kd-tree intersections
Reduce calls of sqrt(..) when finding nearest object, which intersects with ray
Decrease number of recursive calls, when traversing kd-tree - calculating if ray intersects voxel BEFORE recursive call.
Small optimization of function for finding overlapped objects with specific voxel + const + static inline
Small fix in triangle intersection function
Draft optimization of checking if intersection point is inside triangle
Fixed bug: taking into account last light source when shading
Improve function for checking if intersection point is inside of triangle. Using Clockness test (instead of Herons square test). If the three triangles from by each side of the triangle to intersect point have same clocknees as the triangle then point is inside triangle.
Refactoring
Temporarily switched off multithreading
Calculating FPS (using timer signal)
Merge branch 'yura'
Fixed __GNU_PREREQ macro

__GNU_PREREQ() is not supported in apple-gcc, changing it to a more
crossplatform way.