Add support for (GLX|WGL)_EXT_swap_control_tear.

This required a small public API change: SDL_GL_SetSwapInterval() now accepts
 negative values, and SDL_GL_GetSwapInterval() doesn't report errors anymore
 (if it can't work, it'll return zero as a reasonable default).

If you need to test for errors, such as a lack of swap_control_tear support,
 check the results of SDL_GL_SetSwapInterval() when you set your desired
 value.

src/video/windows/SDL_windowsopengl.h

@@ -27,7 +27,8 @@
 
 struct SDL_GLDriverData
 {
-    int WGL_ARB_pixel_format;
+    SDL_bool HAS_WGL_ARB_pixel_format;
+    SDL_bool HAS_WGL_EXT_swap_control_tear;
 
     void *(WINAPI * wglGetProcAddress) (const char *proc);
       HGLRC(WINAPI * wglCreateContext) (HDC hdc);