Support SDF loading through fileIO plugin.
Replace strcspn by C code (not crossplatform)
Add flag for loadURDF to use color from MTL file (instead from URDF link material)
pybullet.URDF_USE_MATERIAL_COLORS_FROM_MTL and pybullet.URDF_USE_MATERIAL_TRANSPARANCY_FROM_MTL
A fileIO plugin can override custom file IO operations. As a small test, load files from a zipfile in memory.
Default fileIO implementation is in examples/Utils/b3BulletDefaultFileIO.h
Affects URDF, SDF, MJCF, Wavefront OBJ, STL, DAE, images.
