Add progress indicator to VPR

The notification area under the Objects, Bones, Lights, and Cameras button now show how fast VPR is rendering. If you have multiple VPR viewports, you will get progress bars for each one:

With one, then two VPR viewports above, but also more:

This meter is switched off by default, so don't be surprised if you don't see these bars right off. Visit Options (O), and at the bottom of the first tab you'll see a new option, Viewport Monitor.