Issue #687: Add OwnedDrawData for deep cloning DrawData

[jbrd]

Introduce an OwnedDrawData struct that holds a cheaply-cloned ImDrawData structure that has been allocated and cloned on the heap. Designed for cases where you need to keep hold of the draw data so it can be evaluated at a later point in time (for instance, in multithreaded renderers where rendering the previous frame is overlapped with updating the next frame). It leverages ImDrawList::CloneOutput for cloning the underlying draw lists, and all allocations are made via ImGui, which has a nice consequence of them being tracked in the IO object's allocation metrics. The OwnedDrawData struct provides an accessor to the underlying DrawData struct, so that it can still be passed to renderers without modifying them.

As discussed in Issue #687

[dbr]

Looks great, thanks!

[jbrd]

Excellent, thank you!