Add toMask and toImage.

src/pixie.nim

@@ -1,3 +1,16 @@
 ## Public interface to you library.
 
-import pixie/common
+import pixie/images, pixie/masks, pixie/paths
+export images, masks, paths
+
+proc toMask*(image: Image): Mask =
+  ## Converts an Image to a Mask.
+  result = newMask(image.width, image.height)
+  for i in 0 ..< image.data.len:
+    result.data[i] = image.data[i].a
+
+proc toImage*(mask: Mask): Image =
+  ## Converts a Mask to Image.
+  result = newImage(mask.width, mask.height)
+  for i in 0 ..< mask.data.len:
+    result.data[i].a = mask.data[i]