Add to_vec function for writing PNG data to a Vec<u8>.

[jgraham]

This mostly copies the implementation of store_png, and could
probably be refactored to share more code in the future.

[metajack]

Can you add a test for this? I figured it would call to_vec and then verify the first bytes were the PNG magic bytes and that the total length was the expected one.

Otherwise, looks fine.

[jgraham]

This is rebased onto master, but with the tests in a separate commit, because they won't trivially backport to the current servo branch (but the actual functionality will).

[metajack]

Squash and r=me.

[metajack]

Or are you saying you want to leave the tests in their own commit? If so, maybe change the commit message to not have fixup! in it?

[jgraham]

I'm happy to squash, but unless the rust upgrade is about to land it would be useful to create a servo branch with 16a9dba cherry-picked on top of the current servo commit.

[metajack]

Seems reasonable then. Just change the commit message to 'Add tests for to_vec.' and I'll merge this.