When {meme ...} embed has no text, just use the raw file data unmodified

Summary:
Ref T13216. See PHI948. When you use the remarkup hint button to embed a meme with no text, you get {meme src=X}.

If the source is a GIF, we currently split the source apart into frame-by-frame images, process them, and stitch them back together. The end result is the same image we started with, but this process can be slow/expensive, and may timeout for sufficiently large GIFs.

Instead: when there's no text, just return the original image data.

Test Plan:

• Used {meme src=X} with no text, got an image faster.
• Used {meme src=X, above=...} to add text, got an attempt to add text (which didn't get very far locally since I don't have GD configured).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19777