default to encoding in the image message (#921)

My camera is publishing rgb8 encoding - the existing code throws an
error that 8UC3 is not a valid encoding, but if we pass rgb8 from the
message then things work fine. The encoding in the image should always
be more descriptive than just the bit and channel size.

If encoding is not filled in, the existing behavior is used as a
fallback

image_view/scripts/extract_images_sync

@@ -105,9 +105,13 @@ Typical command-line usage:
         bridge = cv_bridge.CvBridge()
         for i, imgmsg in enumerate(imgmsgs):
             img = bridge.imgmsg_to_cv2(imgmsg)
-            channels = img.shape[2] if img.ndim == 3 else 1
-            encoding_in = bridge.dtype_with_channels_to_cvtype2(
-                img.dtype, channels)
+            # Default to the encoding in the image message
+            encoding_in = imgmsg.encoding
+            if not encoding_in:
+                # Encoding is not specified, try to automatically determine
+                channels = img.shape[2] if img.ndim == 3 else 1
+                encoding_in = bridge.dtype_with_channels_to_cvtype2(
+                    img.dtype, channels)
             img = cv_bridge.cvtColorForDisplay(
                 img, encoding_in=encoding_in, encoding_out='',
                 do_dynamic_scaling=self.do_dynamic_scaling)