SDL2 does not report keydown and keyup events on modifiers keys until
you release them. Attempted to fix this using SDL_GetModifierState, but
this does not appear to work. Should hit upstream.
This resolves the issue where different layouts would return different
keys when pressing the same physical key. A new API must be introduced
for text input.
This is a new platform that can be used then sdl2 is installed on the target system. SDL2 is commercially supported by Valve and provides better support for MacOS compared to our current implementation (Cocoa vs Carbon). It will also help us introduce faster support for new platforms. Existing platforms remain as a fallback and will be automatically used if sdl2 is not installed.
Please note that this is still a work in progress. The new mouse and keyboard API is not supported yet. Due to limitations of sdl2, multiple mice/keyboards are also not supported.
