Yes, depth for 2D drawing, the idea is for save fill-rate. Enable the depth buffer for 2D so we can draw 1-bit alpha graphics in front-to-back order and save much fill-rate.
2D functions could have a new 'depth%' param (0 used if not indicated) with 256 levels (0-255 values), 0 is total front and 255 is total back. Or use instead the same depth configured with a previous X_MAKE3D.
Using a default value of 0 like with vertex color in POLYVECTORs RGB(255,255,255) with no need to pass a value there is no need to re-write old codes.
Well, there is a way you could do that with code too o:
But with real depth it would be easier for some things, yes.