John/adjust camera to viewport

This branch updates vtkMap to adjust the camera view angle based on the size of the viewport (specifically, the height of the viewport). The current code uses a constant 30 degrees, which is the vtk default. The purpose of this branch is to maintain a 1:1 ratio between the viewport pixel size and the tile pixel size.

By keeping the pixel sizes in sync, texture interpolation is not needed and no longer enabled. The result is a slight improvement in image sharpness, particularly with text/labels.

I am not 100% sure we want to merge these changes, however, as it will introduce perspective distortion of 3-D data (another reason to have 2 renderers, I suppose...)