fix for headless collision shape generation
fix for headless collision shape generation

this commit implements just enough of dummy mesh_storage so collision shapes are still generated in the headless mode
implementation was inspired by rasterizer_dummy.h from Godot3

Valentactive/godot,josempans/godot,Zylann/godot,godotengine/godot,akien-mga/godot,pkowal1982/godot,vkbsb/godot,josempans/godot,Shockblast/godot,guilhermefelipecgs/godot,firefly2442/godot,BastiaanOlij/godot,firefly2442/godot,Valentactive/godot,Shockblast/godot,Zylann/godot,godotengine/godot,Zylann/godot,Shockblast/godot,Zylann/godot,sanikoyes/godot,guilhermefelipecgs/godot,akien-mga/godot,pkowal1982/godot,BastiaanOlij/godot,ZuBsPaCe/godot,akien-mga/godot,sanikoyes/godot,josempans/godot,pkowal1982/godot,Zylann/godot,BastiaanOlij/godot,Valentactive/godot,firefly2442/godot,Valentactive/godot,akien-mga/godot,godotengine/godot,josempans/godot,Valentactive/godot,ZuBsPaCe/godot,sanikoyes/godot,pkowal1982/godot,guilhermefelipecgs/godot,firefly2442/godot,sanikoyes/godot,ZuBsPaCe/godot,godotengine/godot,sanikoyes/godot,sanikoyes/godot,Shockblast/godot,Valentactive/godot,firefly2442/godot,BastiaanOlij/godot,vkbsb/godot,pkowal1982/godot,josempans/godot,BastiaanOlij/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,josempans/godot,ZuBsPaCe/godot,pkowal1982/godot,godotengine/godot,guilhermefelipecgs/godot,sanikoyes/godot,vkbsb/godot,pkowal1982/godot,firefly2442/godot,Zylann/godot,Shockblast/godot,akien-mga/godot,Zylann/godot,guilhermefelipecgs/godot,pkowal1982/godot,vkbsb/godot,akien-mga/godot,Shockblast/godot,akien-mga/godot,vkbsb/godot,ZuBsPaCe/godot,godotengine/godot,BastiaanOlij/godot,vkbsb/godot,akien-mga/godot,guilhermefelipecgs/godot,sanikoyes/godot,firefly2442/godot,Zylann/godot,godotengine/godot,ZuBsPaCe/godot,Shockblast/godot,Valentactive/godot,vkbsb/godot,Valentactive/godot,BastiaanOlij/godot,Shockblast/godot,vkbsb/godot,godotengine/godot,ZuBsPaCe/godot,josempans/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,firefly2442/godot,josempans/godot