GFX file format specification (quickly written, errors inevitable :-)

**MAIN HEADER**
6bytes	"GFX10",1ah
word	number of subfiles
word(s)	paragraph offset to subfile 0,1,2,... (true offset=paraoff*16)

**SUBFILE HEADER**
byte	file type
		0ffh = unknown data
		0feh = palette
		004h = Optimized X sprite
		003h = vertical bitmap
		002h = 1-bitplane packed bitmap
		001h = 4-bitplane packed bitmap
		
**PALETTE**
768 bytes of palette data (I think...)
		
**VERTICAL BITMAPS**
byte	reserved
word	add to Y to center bitmap
word	add to X to center bitmap
word	heigth (pixels)
word	width (pixels)
repeat width times
{
	a=byte
	b=byte
	if(b==0) end of column
	skip (b>>1) pixels
	copy a bytes
}
		
**UNPACKED BITMAP FORMAT**
byte	reserved
word	add to Y to center bitmap
word	add to X to center bitmap
word	heigth (pixels)
word	width (pixels)
byte(s) data from top to bottom, rows from left to right

**1-BITPLANE PACKED BITMAP FORMAT**
byte	reserved
word	add to Y to center bitmap
word	add to X to center bitmap
word	heigth (pixels)
word	width (pixels)
bytes(s) packed data, composed of subblocks, read like this:
for(;;)
{
	a=byte
	if(a==0) end of picture (or bitplane in 4-bitplane mode)
	else if(a>0) skip a bytes
	else if(a&1) copy ((a&127)>>1) bytes to screen
	else b=byte, write b ((a&127)>>1) times to screen
}

**4-BITPLANE PACKED BITMAP FORMAT**
byte	reserved
word	add to Y to center bitmap
word	add to X to center bitmap
word	heigth (pixels)
word	width (pixels)
byte(s) data, like in 1-BITPLANE except that data is saved in
	four passes, one for every tweaked mode bitplane.

**X SPRITE**
byte	reserved (0)
word	add to Y to center bitmap
word	add to X to center bitmap
word	heigth (pixels)
word	width (pixels)
repeat 4 times (bitplane)
{
  repeat heigth times
  {
    repeat until end of row
    {
	b=byte
	if(!b)
	{
		b=byte;
		if(b) skip b 8 byte blocks
		else end of row
	}
	else
	{
		for bits 0..7 in b:
			if bit=0, skip pixel 
			else read byte, write it
	}
    }
  }
  next bitplane
}