Low overhead session replay with runloop observers

[noahsmartin]

Session reply uses a display link to decide when to capture a new frame, this leads to high overhead because even when the UI is not changing session replay is recording a frame. A lower overhead session replay would:

• Only capture a new frame on a new iteration of the run loop. UI is almost only updated from the main run loop, It is possible to have an animation rendering by Core Animation that doesn't use the main runloop.
• Disable recording when the run loop is in interactive mode, to prevent any overhead when scrolling that risks causing frame drops

[linear[bot]]

COCOA-1017