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.

• (send a-bitmap-dc get-argb-pixels x y width height pixels) -> void
     x : real number
     y : real number
     width : exact integer in [1, 10000]
     height : exact integer in [1, 10000]
     pixels : mutable-string

  The pixel RGB values are copied into pixels. The first character represents an alpha value that is currently always 255, the second character represents a red value of the pixel at (x, y), the third character is the blue value, etc. In this way, the first width*height*4 characters of pixels are set to reflect the current pixel values in the DC, where all alpha values are reported as 255. The pixels are in row-major order, left to right then top to bottom.