merged background color with background gradient methods, updated tests to show description of scene

moved some headers to precomp.hpp

fixes based on the feedback: window name prefix is automatically added when it is not there, singleton class returns reference instead of pointer, destructor is private, release function implemented

initial shared pointer implementation for Viz3d, singleton VizAccessor, cv::viz::get function initial implementation

initial implementation of projection matrix, 3D to window coordinates, window coordinates to 3D ray (not yet tested)