R1:c206b066df38

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…