strip metadata for webp and avif

[liudongmiao]

webp and avif is mainly for browser, it's safe to strip metadata.

@cshum Should we add a new option to do so?
Or make filters strip_exif as an option?

[cshum]

It would be better to retain existing behaviour by default,
only strip metadata when the stripExif boolean is true
https://github.com/cshum/imagor/blob/master/vips/process.go#L124

[liudongmiao]

@cshum As there are 3 testcase use strip_exif, and there are exif in the output data, so it's failed.

Could you update the testdata? (Or should I update the testdata in this PR?)

[liudongmiao]

@cshum I remove the strip_exif for other format, and only for webp and avif.

However, for jpeg, the master branch strip metadata always.
And IMO, webp and avif is the similiar for web, so in the first PR, I use the hard code to remove metadata too.

[cshum]

Having thought about it strip_exif is for exif metadata only, but strip metadata is for all metadata.
Should be separated as another feature instead.
Adding filter strip_metadata() and config vips-strip-metadata

#475