(2.4) Backports from master branch (#7854)

PCL PR 1205

This reverts commit f45da986.

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

PCL PR 1205

This reverts commit f45da986.

When I reconstructed the 3D scene I want to show it as viz::WMesh with an initial value of lighting. For this I disable lighting and shadows:
cv::viz::Viz3d viz_3d_window("3D");
viz_3d_window.setRenderingProperty("mesh", cv::viz::LIGHTING, 0);

Most of the examples show the 3D reconstruction using the point cloud, but using  the mesh one part of it creates a shadow on other parts of it if lighting ON.
Using a 3D mesh with disabled lighting gives the most realistic picture of the reconstructed 3D scene.

- 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)

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