Pass required out parameter

After commit d9464d20,
rsvg_decode_data_uri() requires a non-nullable `mime_type` out argument,
instead of allowing it to be NULL.

This means that all callers on the C side need to pass a `mime_type`
argument, even if they will ignore it.
3 jobs from no-mime-type in 6 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #150203
fedora test

00:05:25

 
  Lint
manual #150205
allowed to fail manual
clippy
passed #150204
rustfmt

00:00:55