add 'mass' and 'elasticity' entity properties

src/objects/EntityDef.hpp

@@ -45,6 +45,12 @@ struct EntityDef {
     /// @brief Is the entity solid (blocks other entities)
     bool solid = false;
 
+    /// @brief Mass for physics calculations
+    float mass = 1.0f;
+
+    /// @brief Elasticity for physics calculations
+    float elasticity = 0.0f;
+
     /// @brief Max obstacle height that does not require jumping
     float stepHeight = 0.5f;