Nerf protocol window sizes

Probably due to a unit misunderstanding a long time ago the window sizes
were quite insane (especially the default).
In practice this was sometimes hidden by other bugs, games trying their
best to be lightweight or didn't matter on high-quality internet connections.

src/network/mtp/impl.cpp

@@ -751,6 +751,7 @@ void Channel::UpdateTimers(float dtime)
 			packet_too_late = current_packet_too_late;
 			packets_successful = current_packet_successful;
 
+			// has half the window even been used?
 			if (current_bytes_transfered > (unsigned int) (m_window_size*512/2)) {
 				reasonable_amount_of_data_transmitted = true;
 			}