Add full high DPI information to SDL_DisplayMode

SDL_DisplayMode now includes the pixel size, the screen size and the relationship between the two. For example, a 4K display at 200% scale could have a pixel size of 3840x2160, a screen size of 1920x1080, and a display scale of 2.0.

test/testwm.c

@@ -108,7 +108,7 @@ draw_modes_menu(SDL_Window *window, SDL_Renderer *renderer, SDL_FRect viewport)
         }
 
         (void)SDL_snprintf(text, sizeof text, "%d: %dx%d@%gHz",
-                           i, mode.w, mode.h, mode.refresh_rate);
+                           i, mode.pixel_w, mode.pixel_h, mode.refresh_rate);
 
         /* Update column width */
         text_length = (int)SDL_strlen(text);