Added assert to catch users using Begin/End without a first NewFrame
Added assert to catch users using Begin/End without a first NewFrame

ocornut/imgui,nProtect/imgui,kylawl/imgui,mikesart/imgui,ghassanpl/imgui,gyakoo/imgui,benoitjacquier/imgui,usagi/imguixx,twigletguy/imgui,Psybrus/imgui,sakishum/imgui,cmaughan/imgui,benoitjacquier/imgui,DXsmiley/FlexGUI,twigletguy/imgui,neshume/imgui,redblobgames/imgui,usagi/imguixx,Flix01/imgui,ggtucker/imgui,wasikuss/imgui,cmaughan/imgui,Psybrus/imgui,wflohry/imgui-addons,wangshijin/imgui,thevaber/imgui,thevaber/imgui,burningreggae/imgui,memononen/imgui,gamedeff/imgui,nProtect/imgui,ghassanpl/imgui,Pagghiu/imgui,ggtucker/imgui,bkaradzic/imgui,gyakoo/imgui,ggtucker/imgui,seanmiddleditch/imgui,redblobgames/imgui,Extrawurst/imgui,rmoorman/imgui,shaggytwodope/imgui,xythobuz/imgui,wasikuss/imgui,xunmengfeng/imgui,bagobor/imgui,Pagghiu/imgui,nmlgc/imgui,cmaughan/imgui,thennequin/imgui,tpoechtrager/imgui,shaggytwodope/imgui,luiseduardohdbackup/imgui,burningreggae/imgui,bkaradzic/imgui,thennequin/imgui,nProtect/imgui,andresv/imgui,zho7611/imgui,tom-seddon/imgui,ocornut/imgui,kylawl/imgui,mikesart/imgui,ocornut/imgui,Extrawurst/imgui,zho7611/imgui,luiseduardohdbackup/imgui,sakishum/imgui,bagobor/imgui,aimotive/imgui-glfw,gamedeff/imgui,emoon/imgui,rxantos/imgui,DXsmiley/FlexGUI,Flix01/imgui,rxantos/imgui,emoon/imgui,elect86/imgui,rmoorman/imgui,wangshijin/imgui,seanmiddleditch/imgui,wflohry/imgui-addons,tom-seddon/imgui,GHF/imgui,elect86/imgui,xythobuz/imgui,dougbinks/imgui,neshume/imgui,Flix01/imgui,tpoechtrager/imgui,andresv/imgui,bkaradzic/imgui,xunmengfeng/imgui,cmaughan/imgui,dougbinks/imgui,memononen/imgui,aimotive/imgui-glfw,GHF/imgui,nmlgc/imgui