Object properties: Add 'glow', disables light's effect if negative

The 'glow' value is added to the ambient light value.
Negative 'glow' disables light's effect on object colour, for faking
self-lighting, UI-style entities, or programmatic colouring in mods.

src/content_cao.h

@@ -106,6 +106,7 @@ private:
 	float m_step_distance_counter = 0.0f;
 	u8 m_last_light = 255;
 	bool m_is_visible = false;
+	s8 m_glow = 0;
 
 	std::vector<u16> m_children;