* Support multiple -vf commands
Marge Filters into one command
* Ordering Scale
Using -vf to scale instead of -s to have ordering
* Correct the ResizeFilterTest
* Remove useless comments
* Fixed filter bug
Add checking on filters if only one process.
* Fixed typo error
When the metadata filter is applied on an `Audio` media, the `artwork`
parameter was being unset from the metadata array.
Saving the same `Audio` instance into multiple formats generated the
correct commands for the first export, but the subsequent commands were
missing the `artwork` parameter.
This commit fixes this issue by copying (by value) the `metaArr`
property to a local variable each time the filter is applied.
