Performance issues zooming using the widget

[rpglover64]

Steps to reproduce:

1. Open the pan-zoom widget
2. Use the widget to zoom in and out quickly

Expected:

Zoom occurs almost instantaneously, possibly choppily

Got:

Hoodle's UI freezes for multiple seconds

Hypothesized cause:

Without having looked at the code yet, I'd guess that hoodle is trying to process all the motion events that result from the zoom rather than setting the current zoom level based on the current position of the cursor.