add_image encourages copies of image data

[jrmuizel]

add_image() currently takes a vec this means that if owners of the image data want to keep it around they have to make a copy. This is what currently happens with servo. It would be much better for add_image to take a reference to this data to avoid the copy.

[nical]

Note that ImageData now stores an Arc<Vec<u8>> instead of a Vec<u8> which means servo does not need to copy the image buffer anymore (unless the api sender is in a different process in which case it's a more complicated story).

[kvark]

For IPC, Arc still implements serialization, but the whole vector will be transferred over regardless, so there is not much benefit.

[glennw]

We now have the external image API, which can be used to supply images with zero-copy overhead.