Added AlphaCutout effect

Added AlphaCutout effect, useful for when the player is behind other terrain, but they need to be semi-visible.

• AlphaCutout-1-1-0.c3addon: C3, Create a circular alpha cutout in a texture, specify x,y location of center in the texture (using pixel coordinates), radius (in pixels), alpha value at the center of the cutout and alpha value at the perimeter of the cutout (linear interpolation between the two values.)
• AlphaCutoutDemo-1-0-0.c3p: Example of the AlphaCutout (player visible behind tree sprites.)