8/30/06.  SGX Demo VPC #1

 This is a one of few SuperGrafx VPC demos. This demo shows a simple 
and easy transparency effect. The effect is achieved by setting the
VPC regs $0008/$0009 to $77. The sprite from VDC #1 needs to be set
to low priorty, the same sprite needs to exist in VDC #2 (as high 
priority) and in the same position of the sprite of VDC #1, but 
assigned to an alternate palette (lighte/darker or different HUE/
color). When the two sprites pass infront of the an opaque part of
BG 1, the sprite from VDC #1 will be set behind the BG from VDC #1
 - causing the sprite from VDC #2 to show on top.

 You can control the sprite with the D-pad. Button I cycles through
4 H-sync settings - none (normal), sin wave, VDC #1 BG blur, disable
VDC BG (turn off effect).

 As of this writing the demo only runs on mednafen(0.6.3) with the
SGX option or the real SuperGrafx hardware.

9/16/06.  SGX Demo VPC #2

 This is the second demo in the series of SuperGrafx VPC demos. This demo 
shows more transparency effects through the use of the VPC two 'window'
settings. There are 4 different window configurations; window 1, window 2,
window 1 & 2 (overlap), and no windows. No 'active' windows does not mean the 
display is disabled. You have have up to 3 window configurations in a given
scanline. If changes to the window settings are made during active display,
they will take effect on the following scanline. By changing the width of the
windows on each scanline you can build simple shapes. You can also invert the
shapes too - e.g. end of a stage effect in Super Mario World.

 In this demo I have duplicate sprites and backgrounds for VDC #1 and VDC #2.
One VDC has sprites and background tiles using the normal color palette, while
the other VDC sprites and background tiles uses the alt palette. I set the VPC
to enable VDC #1 display for window 2 only - window overlap and no window have
have only VDC #2 enabled. I also set the priority modes to noraml. None of the 
background tiles use transparent color 0. On an interesting side note: using 
priority mode 10b, you can cut additional holes into the transparency VDC layer.

 Button II switches between transparency modes. Button I toggles 'blur mode'.