(2.4) Backports from master branch (#7854)

Clang's AddressSanitizer throws an alloc-dealloc-mismatch (operator
new[] vs free) error here.

- 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

Added WidgetMerger, Polyline - colors support for each point independently, simple widgets now compute color array instead of setting global color

Conflicts:
	modules/viz/src/clouds.cpp