An empty buffer is a perfectly valid construct that may not be asserted upon. The 3D scene does not use indices so the buffer receives no data and remains empty.
This made the softpoly renderer fail in debug builds. Performance issues aside it works fine now.
Fixes#314
Most of what got added is still unused.
# Conflicts:
# source/build/src/palette.cpp
# Conflicts:
# source/build/src/palette.cpp
# Conflicts:
# source/common/engine/i_interface.h
