Diff viewer - Don't zoom out the geometry windows when changing the diff type.

[SrNetoChan]

Is your feature request related to a problem? Please describe.
Some time one needs to use different diff types to check the introduced changes. Currently, every time you change the diff type, the window will zoom out to extent of the geometry, making you loose the area that you were inspecting

Describe the solution you'd like
It would be nicer not to trigger the zoom to feature extent when the diff type is changes. If needed a new button on context menu could be added to perform that zoom to extent.

[rcoup]

thoughts @hamishcampbell @pfw?

I wonder if it should be a behaviour toggle? And some sort of "your feature isn't fully visible on the map " message.

[pfw]

I've run into this when you have a feature like a pipeline and a single vertex has changed, I wonder how complex it would be to isolate the changed vertexes and use an envelope of those to set the canvas extent. Even with that you would need a 'zoom to full feature' button of some kind, it could be done either way, default to view all and 'zoom to changes', etc.