Remote resources support.

There is now a PRELOAD_RSC define in code/_compile_options.dm. Setting it to 0 will allow you to use on-the-fly rsc downloads, or the new remote rsc features.

Normally (PRELOAD_RSC == 1) the game will send you all the resources for the game that existed at compile time, this is the long-delay before joining the game...but it means that the game isn't sluggish later on due to trying to download icons and such. Which is good, but uses a lot of bandwidth.

On-the-fly behaviour (PRELOAD_RSC == 0) is when the game only downloads icons when you see something for the first time...this is often responsible for things being briefly invisible on slow connections.

Remote RSC behaviour (PRELOAD_RSC == 0 and some urls defined in config/external_rsc_urls.txt), is a mixture of the two above behaviours. It allows you to connect without downloading lots of resources immediately. However, once you connect it will select a url from a list of urls which hold zipped up copies of the tgstation.rsc. This allows the load of downloading those large files to be distributed across a few cheap web-servers or free upload sites...whilst the main game-server is freed up for other stuff. Should preloading from a remote url fail, behavior will revert to on-the-fly.