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Creates an AlphaBitmap out of a standard RGB bitmap and a 'transparency layer'.

AlphaBitmap _CRalphaBitmap ( Chn _channel_ ObjBitmap _bitmapRGB_ ObjBitmap8 _bitmapAlpha_ I _background_ I _transparency_ )

_channel_
	the channel in which the object is created
_bitmapRGB_
	the classical description of the RGB components of the image
_bitmapAlpha_
	the description of the alpha layer, each pixel color of the ObjBitmap8 corresponds to a level of transparency between 0 and 255
_background_
	the object's background color value (might be nil)
_transparency_
	the object's transparency color value (might be nil)

The '_bitmapRGB_' and the '_bitmapAlpha_' bitmaps must have the same size (height and width). Using objects of different sizes does not cause an error during creation but copying (_CPalphaBitmap and _SCPalphaBitmap) will be limited to the area common to both objects.

If '_bitmapAlpha_' is set to nil, the alphaBitmap obtained is nonetheless valid and can be used in all operations.
The transparency color takes priority over the value of the alpha layer when copying.

AlphaBitmap, _LDalphaBitmap, _DSalphaBitmap.