HighDPI support + odds and ends

This patch adds support for the SDL HighDPI stuff (for MacOSX Retina
displays et al), as well as adding a few missing things from SDL 2.0.0
and SDL 2.0.1:
* HighDPI support (hint, window flag, SDL_GL_GetDrawableSize)
* SDL_TICKS_PASSED() macro
* SDL_GL_GetCurrentWindow()
* SDL_GL_GetCurrentContext()
* SDL_HINT_RENDER_DIRECT3D_THREADSAFE
* SDL_HINT_TIMER_RESOLUTION
Merge pull request #45 from sulix/master

HighDPI support + odds and ends