MPictureDissolveIn PictureBitmapNum.w,Colour.b ; Do a picture-based colour-number dissolve-in of a bitmap

Performs a picture-based dissolve in order to `bring in' an source image onto a
destination bitmap. You first have to `use' two bitmaps with MUseBitmap, in
which you specify the source and destination chunky Bitmap's. Then you specify
the `Picture Bitmap' as a parameter to this command which will be the number of a
chunky Bitmap containing an `effect' picture. The routine will search for the
location of pixels in the effect picture that are of the specified colour. When
it finds one it will copy a pixel from those coordinates in the source Bitmap to
the same coordinates in the destination Bitmap. Note that all three Bitmaps in
this operation have to be the same size. To fully bring-in a whole picture you
should call this command as many times as there are colours in the effect-picture
so that all pixels are eventually copied. The more colours you make use of in the
effect-picture the longer the dissolve will take. Note that the effect-picture
should still be 256-colours even if you only use 64, for example. To create an
effect-picture that produces a nice screenwipe you should create a simple
greyscale gradient in something like Deluxe Paint and then perform some good
image-processing operations on it in something like ImageFX to distort and mangle
the gradient to bring variety into the effect. Note that if the destination
Bitmap already contains an image and you are dissolving in a new one, the two
images should use the same colour palette otherwise one of the two images will
appear in the wrong colours. If the destination bitmap starts off blank it will
just cause the source image to gradually appear on, for example, a black
background.