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 Screen Descriptor

 The Screen Descriptor describes the overall parameters for all  GIF images
 following.  It defines the overall dimensions of the image space or
 logical screen required, the existance of colour mapping  information,
 background  screen colour, and colour depth information.  This information
 is stored in a series of 8-bit bytes as described below.

              bits
         7 6 5 4 3 2 1 0  Byte #
        +---------------+
        |               |  1
        | Screen Width  |      Raster width in pixels (LSB first)
        |               |  2
        |---------------|
        |               |  3
        | Screen Height |      Raster height in pixels (LSB first)
        |               |  4
        |---------------|      M = 1, Global colour map follows Descriptor
        |M|  cr |0|pixel|  5   cr+1 = # bits of colour resolution
        |---------------|      pixel+1 = # bits/pixel in image
        |   background  |  6   background=colour index of screen background
        |---------------|          (colour is defined from the Global colour
        |0 0 0 0 0 0 0 0|  7        map or default map if none specified)
        +---------------+


 The logical screen width and height can both  be  larger  than  the
 physical  display.   How  images  larger  than  the physical display are
 handled is implementation dependent and can take advantage  of  hardware
 characteristics  (e.g.   Macintosh scrolling windows).  Otherwise images
 can be clipped to the edges of the display.

 The value of 'pixel' also defines  the  maximum  number  of  colours within
 an  image.   The  range  of  values  for 'pixel' is 0 to 7 which
 represents 1 to 8 bits.  This translates to a range of 2 (B & W) to  256
 colours.   Bit  3 of word 5 is reserved for future definition and must be
 zero.