Various fixes to ugoira->webm converter

* Fix regex for getting frame data not working if agent is logged in to
pixiv
* Fix #unpack not returning the unpacked folder
* Fix frame_data being inaccessible because it was a local variable
* Fix images getting put in tmpdir, when they're expected to be in
tmpdir/images
* Fix deleting the tmpdir manually (it gets deleted automatically)
* Fix output webm being put in tmpdir (which gets deleted) instead of
write_path
Fix last frame of webms not displaying right in some browsers

Some browsers like Chrome (and some video players like MPC-HC) do not
display the last frame of a webm (or they display it, but only very
briefly) if set to loop. This commit is a hack to force the last frame
to display for the correct amount of time by duplicating it.