video: Select the closest mode for the target refresh rate accounting for small variations

Use the delta of the target refresh rate and the refresh rate of the mode to select the closest matching display mode, as the actual closest mode may have a slightly lower rate than the target by a small fraction of a hz (e.g 59.98 vs 60.0).

src/video/SDL_video.c

@@ -1055,8 +1055,8 @@ const SDL_DisplayMode *SDL_GetClosestFullscreenDisplayMode(SDL_DisplayID display
                     continue;
                 }
 
-                if (mode->refresh_rate < refresh_rate) {
-                    /* We already found a mode and the new mode doesn't meet our
+                if (SDL_fabsf(closest->refresh_rate - refresh_rate) < SDL_fabsf(mode->refresh_rate - refresh_rate)) {
+                    /* We already found a mode and the new mode is further from our
                      * refresh rate target */
                     continue;
                 }