Render interface consistency

These changes mostly represent changing the interface to some of the rendering classes to be more consistent with the reset of VTK-m. In particular, several exposed class members become private and are accessed through methods.

There are some other changes to make the interface simpler to use. One example is consolidating the width/height of the image to the Canvas. (All other classes refer to the Canvas for the width/height). Another example is removing the template from the View class.