set-argb-pixels in bitmap-dc%

Sets a rectangle of pixels in the bitmap, subject to the same rules and performance characteristics of set-pixel, except that the block set is likely to be faster than the sequence of individual sets.

• (send a-bitmap-dc set-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 : byte string
     alpha? = #f : boolean

  The pixel RGB values are taken from pixels. The first byte represents an alpha value, the second byte represents a red value to used for the pixel at (x, y), the third byte is a blue value, etc. In this way, the first width*height*4 bytes of pixels determine the new pixel values in the DC. The pixels are in row-major order, left to right then top to bottom.

  If alpha? is false, then the alpha value for each pixel is ignored. If alpha? is true, then only the each pixel is set based only on the alpha value. Thus, the same pixels byte string is in general used with two bitmaps, one (the main image) for the pixel values and one (the mask) for the alpha values.