Redundant call to render function in vdb viewer

https://github.com/dreamworksanimation/openvdb/blob/ffb02d5527379a08c07449e39d3324ac2e5eff28/openvdb/viewer/Viewer.cc#L1159

The `mUpdates` variable is reset at every key callback. However, I believe `mUpdates` should be reset only when key is pressed. Presently this variable is reset when the key is pressed as well as when it released. Due to this, the `render` function is called four times instead of two.

Open openvdb/viewer/Viewer.cc around line 1159 and trace the key callback's handling of mUpdates and render. Verify the behavior for key press and release, then confirm the viewer renders twice rather than four times for the reported interaction.

Written by the indexing model from the issue text.