Camera: IntrinsicsPinhole convenience upgrades

[orperel]

Additions to IntrinsicsPinhole:

1. Add properties for cx and cy, which represent the actual principal points (currently we expose only x0, y0, which are deltas from the center.
2. Add a matrix for casting from NDC to pixel space. This should be the matrix form of glViewport , to be used with the ViewProjection matrix when going from world coordinates to actual pixel coordinates (rather than world to NDC).
3. Upgrade setter functions for the width and height properties: these should maintain the focal length invariant, but update the fov accordingly

Additions to Camera:

1. Expose setter functions for the width and height properties.
2. Add pin_memory() function.