get-argb-pixels in bitmap%

Produces the same result as get-pixel in bitmap-dc% with the bitmap selected into the DC, but the bitmap does not have to be selected anywhere (and this method works even if the bitmap is selected into another DC, attached as a button label, etc.).

• (send a-bitmap get-argb-pixels x y width height pixels alpha?) -> void
     x : real number
     y : real number
     width : exact integer in [1, 10000]
     height : exact integer in [1, 10000]
     pixels : mutable byte string
     alpha? = #f : boolean