Add write_images_blob

This method goes down to `MagickGetImagesBlob`, which allows for
creating animated gifs.

I didn't abstract out common operations between the two in the interest
of not overly polluting the codebase. However, this new method is almost
identical to `write_image_blob` so you could probably abstract something
out of there.

src/wand/magick.rs

@@ -218,6 +218,27 @@ impl MagickWand {
         Ok(bytes)
     }
 
+    /// Write the images in the desired format to a new blob.
+    ///
+    /// The `format` argument may be any ImageMagick supported image
+    /// format (e.g. GIF, JPEG, PNG, etc).
+    pub fn write_images_blob(&self, format: &str) -> Result<Vec<u8>, &'static str> {
+        let c_format = CString::new(format).unwrap();
+        let mut length: size_t = 0;
+        let blob = unsafe {
+            bindings::MagickSetImageIndex(self.wand, 0);
+            bindings::MagickSetImageFormat(self.wand, c_format.as_ptr());
+            bindings::MagickGetImagesBlob(self.wand, &mut length)
+        };
+        let mut bytes = Vec::with_capacity(length as usize);
+        unsafe {
+            bytes.set_len(length as usize);
+            ptr::copy_nonoverlapping(blob, bytes.as_mut_ptr(), length as usize);
+            bindings::MagickRelinquishMemory(blob as *mut c_void);
+        };
+        Ok(bytes)
+    }
+
     string_set_get!(
         get_filename,                    set_filename,                    MagickGetFilename,                 MagickSetFilename
         get_font,                        set_font,                        MagickGetFont,                     MagickSetFont

tests/lib.rs

@@ -82,7 +82,7 @@ fn test_read_from_blob() {
 }
 
 #[test]
-fn test_write_to_blob() {
+fn test_write_image_to_blob() {
     START.call_once(|| {
         magick_wand_genesis();
     });
@@ -103,6 +103,28 @@ fn test_write_to_blob() {
     assert_eq!(384, wand.get_image_height());
 }
 
+#[test]
+fn test_write_images_to_blob() {
+    START.call_once(|| {
+        magick_wand_genesis();
+    });
+    let wand = MagickWand::new();
+    assert!(wand.read_image("tests/data/IMG_5745.JPG").is_ok());
+    assert_eq!(512, wand.get_image_width());
+    assert_eq!(384, wand.get_image_height());
+    let blob = wand.write_images_blob("jpeg").unwrap();
+    if cfg!(target_os = "macos") {
+        // yeah, don't know why...
+        assert_eq!(104061, blob.len());
+    } else {
+        assert_eq!(104060, blob.len());
+    }
+    // should be able to read it back again
+    assert!(wand.read_image_blob(&blob).is_ok());
+    assert_eq!(512, wand.get_image_width());
+    assert_eq!(384, wand.get_image_height());
+}
+
 #[test]
 fn test_fit() {
     START.call_once(|| {