GFX condact support for DAAD

[jlcebrian]

GFX condact is used extensively in Chichen (which looks broken without it), and up to some extent in Espacial and Templos. It's actually not hard to support and could be very useful for future adventures too.

Here is what it needs:

• There are actually two graphic screens (the front buffer and the back buffer)
• The front buffer is the one displayed to the user
• By default, all drawing operations (clearing, scrolling, pictures, text) go to the front buffer, but this can be configured
• GFX n 0 copies the back buffer to the front buffer
• GFX n 1 copies the front buffer to the back buffer
• GFX n 2 swaps the contents of both buffers
• GFX n 3 sets picture drawing to go to the front buffer
• GFX n 4 sets picture drawing to go to the back buffer
• GFX n 5 clears the front buffer using n as color
• GFX n 6 clears the back buffer using n as color
• GFX n 7 sets text output to go to the front buffer
• GFX n 8 sets text output to go to the back buffer
• GFX n 9 changes a palette color: 'n' is a flag containing the color number, flags n+1 to n+3 contains R, G, B values
• GFX n 10 gets the value of a palette color (same parameters as GFX n 9)

Color values are in 0-255 range but the number of bits supported by varies by hardware (4 for ST, 5 STe, 6 VGA for example).