Show a basic storage type to handle a simple RGB raster graphics image,
and some primitive associated functions.

If possible provide a function to allocate an uninitialised image,
given its width and height, and provide 3 additional functions:

   one to fill an image with a plain RGB color,
   one to set a given pixel with a color,
   one to get the color of a pixel.


(If there are specificities about the storage or the allocation, explain those.)

These functions are used as a base for the articles in the category raster graphics operations, 
and a basic output function to check the results
is available in the article write ppm file.




procedure makebitmap(width,height)
   return open("bitmap", "g", "canvas=hidden",
             "size="||width||","||height)
end
procedure fillimage(w,color)
   Fg(w,color)
   FillRectangle(w)
end
procedure setpixel(w,x,y,color)
   Fg(w,color)
   DrawPixel(x,y)
end
procedure getpixel(w,x,y)
   return Pixel(w,x,y)
end