cell: handle NULL cso vertex_elements state object on bind
cell: handle NULL cso vertex_elements state object on bind

drivers need to handle NULL cso vertex elements (and others) objects.
It is possible the cso code saves/restores NULL objects (if no normal
cso object was bound before this was invoked).
This led to segfaults (for example demos/cubemap) for apps which were using
things like creating mipmaps before drawing anything.

zz85/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,adobe/glsl2agal,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,KTXSoftware/glsl2agal,dellis1972/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,adobe/glsl2agal,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,KTXSoftware/glsl2agal,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,bkaradzic/glsl-optimizer,adobe/glsl2agal,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,adobe/glsl2agal,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer