Renderer: Don't sort z=0 Commands

If Command has z==0, then those elements won't be sorted.
Only Z !=0 will be sorted, and it will use `sort` instead of `stable_sort`
for z!=0, since it is faster
Merge pull request #5120 from ricardoquesada/renderer_no_sort_z_0

Renderer: Don't sort z=0 Commands