changed drawing preview/network players if only level preview redefined

Problem: Drawing preview players (for team mode levels) and network
players (when connected to network server) in the main menu was added
at a time where already quite some custom artwork was created, which
did not know of these artwork definitions, so redefining the level
preview area might conflict with those player graphics added later
(and therefore maybe not redefined by these custom artwork sets).

Solution: If the position of the level preview is redefined, but the
positions of the preview or network player graphics are not redefined,
just skip drawing these player graphics.