After committing #680 for supporting SDL_GL and not using SDL_GetWindowWMInfo() anymore, including SDL_syswm.h (which is a Windows-only include file) is no longer needed. This bit had been forgotten in the previous PR.
Migrate to SDL_GL functions made available in SDL2. Remove wgl_ext.c and wgl_ext.h.
Add additional information to the log file to help debug issues like #582.
As ChocolateFan once explained on Discord, in C functions declared as
parameterless in fact are free to take arbitrary number of arguments.
To tell the compiler that the function is not allowed to take any
arguments, we use "func(void)" idiom.
