update Makefile to use SDL2 libraries
make it compile
get rid of deprecated SDL_ListModes, use SDL_GetDisplayMode
don't forget to initialize OpenGL context
fix Neverputt
don't read uninitialized values when changing resolution

When changing the window resolution, the game state is changed to
'null', calling resol_leave() and free'ing the mode-list. Then its value
is passed to video_mode(), resulting in garbage and wrong window size.
The width and height values are saved to local temporary variables
before changing the state, thus passed values are now correct.

This partially fixes issue #3.
destroy window and GL context on mode-setting

When graphical settings are changed, video_mode() is called. Another
window was created, without destroying the old one. This is now fixed.
GL context is also tracked and destroyed when needed.

This fixes issues #1 and #3.
enable relative mouse mode on window grab

When grabbing a window, the mouse move was limited by the window size.
Enabling relative mouse move solves this problem.
Compile as C99 due to SDL2 headers being incompatible with ANSI C
Fix leftover include
Remove X11 RGBA window icon code, SDL2 finally has that
Remove more sync leftovers
Replace SDL_ACTIVEEVENT
Implement text input using SDL2 API
Get rid of leftover static linkage bits
Fix warnings
Fix key name pointer caching
Remove and reuse
Key name to integer translation
Remove unused module