Discord Rich Presence

Adds Discord Rich Presence support. Can be disabled with the HAVE_DISCORDRPC compile option. This opens up some fun options, like...

• Being able to show what you're doing in-game
• The ability to invite a Discord channel to your current game
• The ability to ask a Discord user to be able to join their current game

CVars are as follows:

• discordrp: Enables/disables Rich Presence entirely. All it will give to Discord is an icon saying that you're playing SRB2Kart. (If that's too much, then you probably just want to disable Game Activity entirely :V)
• discordstreamer: Enables/disables "Streamer Mode". When this is turned on, any "discriminators" (the little number tag after names) will not be shown at all.
• discordasks: Client-side option that determines if YOU can send Discord game invites or not. Turn this off if you do not want an "Ask to Join" button on your profile.
• discordinvites: Server-side option that determines if CONNECTED PLAYERS can send Discord game invites or not. Values are "Everyone" and "Admins Only".

All of these options are in the menu, under [Options -> Data Options -> Discord Options].

The Ask to Join feature has in-game support too! Go to the pause menu when you get a request (indicated by the flashing envelope icon and Sonic 3 "EH" sample :P), and it will show you all of your requests. Press Accel/Enter to accept a request, Brake/Escape to decline one, then it'll give you the next request in the list.

MUST BE TESTED:

• Can builds be compiled with HAVE_DISCORDRPC on all targets? (Windows, Linux, Mac... 32-bit, 64-bit)
• Are builds compiled with HAVE_DISCORDRPC able to connect to servers running builds without, and vice versa? Rich Presence should NOT be a requirement for being able to play the game online!
• I need people to triple check to make sure I added a new optional dependency correctly, This is the first time I ever done anything like this to SRB2's source code

Assets required: discord_gfx.wad