ContentCAO: Fix threshold of alpha channel textures (#14213)

With disabled shaders, the material EMT_TRANSPARENT_ALPHA_CHANNEL uses the
parameter as an alpha threshold to decide whether to draw the texture.
Thus lowering this limit fixes the issue of vanishing textures below alpha 128.

src/client/content_cao.h

@@ -130,6 +130,7 @@ private:
 	bool m_is_visible = false;
 	// Material
 	video::E_MATERIAL_TYPE m_material_type;
+	f32 m_material_type_param;
 	// Settings
 	bool m_enable_shaders = false;