Having the path to each shader is helpful when debugging the
rendering pipeline. The render command encoder label is limited
to just the filename of the shader, as the graphics debugging
UI in Xcode doesn't handle long names very well.
We don't encode the pass number into the string as the shaders
can be organized in a graph, not just a linked list, so an
indexed order can be misrepresenting the flow.
b5cf9b639e