Several changes to improve zooming behaviour.

[rdvincent]

This collects several small changes. One of the main goals is to make the volume viewer more usable with smaller field-of-view images such as mouse brains.

1. For the volume viewer demo, use the 'r' key to reset the view.
2. When resetting, reset zoom to the computed "preferred" or default value.
3. Compute the preferred zoom when the volume is loaded.
4. Draw the cursor with a fixed line width, so that it doesn't get absurdly thick at high zoom levels.
5. Use multiplicative rather than additive calculation when zooming, to improve responsiveness at high zoom levels.