rsvg: Determine if svg from first 4096 bytes
rsvg: Determine if svg from first 4096 bytes

The previous value of 128 isn't sufficient because some hand-crafted
SVGs have comments at the start. Going for a whole page of memory seemed
like the right amount of data without becoming too slow.

autrimpo/imv,autrimpo/imv,eXeC64/imv,eXeC64/imv