Use interfaces wherever it's possible in :api and :engine

api/src/main/java/com/bartlomiejpluta/base/api/game/entity/Entity.java

@@ -6,6 +6,8 @@ import com.bartlomiejpluta.base.api.internal.render.Renderable;
 import org.joml.Vector2i;
 
 public interface Entity extends Placeable, Renderable, Updatable {
+   void setStepSize(float x, float y);
+
    Vector2i getCoordinates();
 
    void setCoordinates(Vector2i coordinates);

api/src/main/java/com/bartlomiejpluta/base/api/game/image/Image.java

@@ -13,4 +13,8 @@ public interface Image extends Placeable, Renderable {
    int getWidth();
 
    int getHeight();
+
+   void setOpacity(float opacity);
+
+   float getOpacity();
 }