ImDrawData: changed CmdLists from raw array to ImVector<> owned by ImDrawData itself. Faclitate user-manipulation of the array (#6406, #4879, #1878) + deep swap. (#6597, #6475, #6167, #5776, #5109, #4763, #3515, #1860)

+ Metrics: avoid misleadingly iterating all layers of DrawDataBuilder as everything is flattened into Layers[0] at this point.

# Conflicts:
#	imgui.cpp
#	imgui_internal.h