This command will add a line to the current batch. The amount of lines added is monitored and will not add any past the maximum set.
Specifying a colour will override the colour set by D3D_COLOR. Specifying two colours will create a gradient from Colour1 to Colour2.
The batch must be initialised with D3D_BATCH_SET_LINE2D and
must be drawn with D3D_BATCH_DRAW_LINE2D.
D3D_BATCH_ADD_LINE2D X1, Y1, X2, Y2
D3D_BATCH_ADD_LINE2D X1, Y1, X2, Y2, Colour
D3D_BATCH_ADD_LINE2D X1, Y1, X2, Y2, Colour1, Colour2