fixed receiving responses from score server

fixed receiving responses from score server

This commit fixes problems with receiving (slightly) larger responses
from the score server, causing high score lists with many entries to
be cut off after a few dozen entries.

This bug was caused by misleading documentation of SDL_net function
"SDLNet_TCP_Recv()", which claims to "wait until the full requested
length is sent", which unfortunately is not correct. Instead, data
sent from the server has to be polled until everything is completely
transmitted (using the "Content-Length" field in the HTTP header).