add primitive depth

Author: HeliumAnt

Source/System/Constants.h

@@ -144,6 +144,7 @@ namespace RTE {
 	    -1.0f, -1.0f, 0.0f, 1.0f};
 
 	static constexpr float c_GuiDepth = -100.0f;
+	static constexpr float c_PrimitiveDepth = -75.0f;
 	static constexpr float c_DefaultDrawDepth = 0.0f;
 	static constexpr float c_TerrainBGDepth = 50.0f;
 	static constexpr float c_BackgroundDepth = 100.0f;