Blender Git Loki

Fix missing icons in non-portable local builds.

The problem is that some local path is always returned by
bpy_resource_path. The function does not enforce a check for existence
of the path in the low level code.

Since client code may indeed not care about existence of the directory,
I leave bpy_resource_path alone and patch the python code to check for
existence. An extra argument that enforces the check might be a better
solution, but I will be leaving this to the UI team.