Makefile[.*]: SDL2 build support. 'make USE_SDL2=1' to enable.


git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1019 af15c1b1-3010-417e-b628-4374ebc0bcbd
Revamp SDL2 key handling, based on how ioquake3 does it.

- Reverted the addition of the interpret_shift param to Key_Event
- Now, SDL_KEYUP/SDL_KEYDOWN are converted from SDL scancodes to quake keys, and passed directly to Key_Event with no extra conditions / logic
- When SDL2 is used, Key_Console/Key_Message no longer convert keys to characters to insert in the typing buffer, but all other functionality is unchanged
- SDL_TEXTINPUT calls a new function, Char_Event. This calls either Char_Console/Char_Message depending on the keydest. These just insert the typed text into the buffer

This is cleaner than before because it maintains a clear separation between key presses and characters, whereas the old code was converting characters from SDL_TEXTINPUT back into fake keys and then feeding those into Key_Event. This also fixes Ctrl+C / Ctrl+V not working in the console

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk@1020 af15c1b1-3010-417e-b628-4374ebc0bcbd