Iterate over all displays to create screenshot

[bmhenry]

Uses scrap::Display::all to iterate over all available displays and construct a large buffer to hold all the BGRA structured pixels before referencing that in the screenshot ImgRef.

This likely needs review to (at least) move the timer commands if I misplaced them when adding code.

[owenthewizard]

Iirc when I worked on this way back, XCB could capture the entire screen (all monitors) at once, without needing to allocate memory to stitch them together manually, unless it was doing that behind the scenes anyway.
If possible I'd like to avoid the extra allocations.
Haven't looked into yet (not back until next week), but if scrap doesn't expose it I might need to go back to xcb FFI.