Fix bug where GIF input would error on output.

Also minor corrections to jsdocs in ImageManipulation

src/core/operations/CropImage.mjs

@@ -116,7 +116,12 @@ class CropImage extends Operation {
                 image.crop(xPos, yPos, width, height);
             }
 
-            const imageBuffer = await image.getBufferAsync(jimp.AUTO);
+            let imageBuffer;
+            if (image.getMIME() === "image/gif") {
+                imageBuffer = await image.getBufferAsync(jimp.MIME_PNG);
+            } else {
+                imageBuffer = await image.getBufferAsync(jimp.AUTO);
+            }
             return [...imageBuffer];
         } catch (err) {
             throw new OperationError(`Error cropping image. (${err})`);