Fix logic in the GIF LOOP-detection parsing

We align with Blink/Chromium code by:
  - checking for ANIMEXTS1.0 signature too
  - using ByteCount >= 3 instead of requiring ByteCount==3

Change-Id: Idc484ca62878517df3dccb1fdb3bb45104a5e066
see: http://odur.let.rug.nl/kleiweg/gif/netscape.html