Vtk shaders

Starting merge request for vtk shaders, pulled from ricardo branch, implemented mostly by tansel, rebased on master, with cleaning and corrections made by me.

This is a Work in Progress. do not merge. We need to review this altogether with @ricardo-ortiz

This deprecates an old texture handling, but we need to work on integrating the new implementation, for example if there are no shaders. Also maybe look into getting rid of the use of static objects.

@tanselhalic could you maybe give a description of your work on the shaders and the textures? Also please wait until this is merged to contribute your new changes, so we can start from a clean base again.

Thank you all. Let's get this merged!