Feature request: Semi-automatic reduction of captures

[mstange]

WebRender captures can be rather large, which makes them hard to debug.

It could be nice to have a tool that tries to reduce the complexity of a capture automatically, possibly with user input of the form "is the issue still present?".

[jrmuizel]

It can be difficult for a user to answer this without knowing how the capture has been mutated.

[glennw]

Related to #2475

[nical]

I would love to have a way to take a capture and remove all elements that outside a certain rect (typically, capture a bug in firefox and remove the chrome and other elements around the bug from the capture).