INT FC - DoubleDOS - GET CURRENT SCREEN BUFFER ADDRESS                          
Return: ES = segment of display buffer
Desc:	determine the address of the virtual screen to which the program
	  should write instead of the actual video memory, so that the
	  multitasked programs do not interfere with each other's output
Note:	the display buffer may be moved if multitasking is enabled
SeeAlso: INT 21/AH=ECh"DoubleDOS",INT FB"DoubleDOS"