Implement hitstop shake (#42)

Reviewed-on: #42
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/attack/mod.rs

@@ -82,6 +82,10 @@ impl HitStop {
 
         Self(frames)
     }
+
+    pub fn calculate_shake_frames(&self, game_settings: &GameSettings) -> u32 {
+        (FGi32::from_num(self.0) * game_settings.fighter_common.visual_shake_frames_per_hitstop_frame).floor().to_num()
+    }
 }
 
 #[derive(Clone, Copy, Debug, Serialize, Deserialize)]