Internals: added ImSpanAllocator<> helper.
Internals: added ImSpanAllocator<> helper.

ggtucker/imgui,Flix01/imgui,bkaradzic/imgui,nProtect/imgui,Flix01/imgui,ggtucker/imgui,ocornut/imgui,bkaradzic/imgui,nProtect/imgui,nProtect/imgui,ggtucker/imgui,ocornut/imgui,ocornut/imgui,Flix01/imgui,bkaradzic/imgui