get-argb-pixels in bitmap-dc%

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

The bitmap% class provides this method directly, so it is not necessary to select a bitmap into a DC to extracts its pixel values.

• (send a-bitmap-dc 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

  The pixel RGB values are copied into pixels. The first byte represents an alpha value of the pixel at (x, y), the second byte represents a red value of the pixel at (x, y), the third byte is the blue value, etc. In this way, the first width*height*4 bytes of pixels are set to reflect the current 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 set to 255. If alpha? is true, then only the alpha value is set for each pixel, based on each pixel's value. Thus, the same pixels byte string is in general filled from two bitmaps: one (the main image) for the pixel values and one (the mask) for the alpha values.