Piccu Engine v1.3.0-proto2 SDL

Piccu Engine prototype release, using SDL backend.
Experimental, is probably buggy.

Changes

• All input, window management, and graphics context management is now done through SDL.
• In multiplayer games, when you change which weapon slot is selected, you will always pick the higher priority weapon for that slot.
• Fix pressing esc at pilot screen causing controls to not load.
• Fix terrain fog not rendering in Compat profile.
• Fix ship selector not being visible in Core profile.
• Fix screenshots of 640x480 screens being cut off.
• Fix homing missile warnings not working for a while when changing levels.
• Fix slow movement when keyboard ramping is exactly 0.

Known issues

• Dedicated server not supported yet.
• Sounds may cut off in some levels due to attempt to fix fusion cannon sound vanishing. Further work is needed.
• Some config won't import from older versions of Piccu Engine (all controls will port over just fine), and config file isn't portable yet.