Originally the bindings object in command.h created 5 std::unordered_map for the vertex and 5 for the fragment stage. The deallocation of these maps adds measurable overhead to render pass encoding. In flutter/engine#44990 and flutter/engine#45021 I reduced the number of maps from 5 to 2. But since most cmds will 0-1 bound items, we can be more efficient if we slightly inline the map: that is have a slot on the binding object for id 0, and the a ptr to a map that we lazily create if we need more