INT 5F - HP 95LX GRAPHICS PRIMITIVES - GET IMAGE
        AH = 0Dh
        DX,CX = row,column of first corner
        BP,SI = row,column of second corner
        ES:DI -> image buffer (see below)
Note:   the specified corners are included in the saved image

Format of image buffer:
Offset  Size    Description
 00h    WORD    number of planes (always 01h on HP 95LX)
 02h    WORD    number of bits/pixel (always 01h on HP 95LX)
 04h    WORD    image width in pixels
 06h    WORD    image height in pixels
 08h  N BYTEs   image data
                requires (WIDTH+7)/8 * HEIGHT bytes