Fix #114685: GP time offset modifier doesn't work if the animation is 1 frame long

This is due to the early return of current frame when start and end of
custom range is same

source/blender/gpencil_modifiers_legacy/intern/MOD_gpencil_legacy_time.cc

@@ -102,7 +102,7 @@ static int remap_time(GpencilModifierData *md,
     offset = abs(efra - sfra + offset + 1);
   }
   /* Avoid inverse ranges. */
-  if (efra <= sfra) {
+  if (efra < sfra) {
     return cfra;
   }