getViewerPose() doesn't modify an object of the class so it can be
made const. It also makes this method consistent with other getters
in the class as they are defined as const.

- load OBJ file
- set offscreen rendering
- set roll angle of the camera
- get Mat screenshot of the current scene
- remove all lights from the scene
- add a custom light in the scene
- modify the size of the WImage3D widget
- added ambient property for the widget

Changed Vec3d in cv::viz::Color

Renamed method getMatScreenshotin getScreenshot

Modified showWidget

Fixed on viz::Color and reverted fix on vtkProp3D

Removed cameraRoll method

Merged load mesh method (for ply and obj file)

Fixed doc

Fixed cv::viz::WImage3D::setSize for vtk5.8

Fixed enum for cv::viz::Mesh::load

Should fix #4191 (http://code.opencv.org/issues/4191)

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

moved some headers to precomp.hpp