Allow setting a custom pixel density in the JPEG encoder

[lovasoa]

This pull request allows setting a custom DPI value for JPEG images (the encoder used to hardcode a pixel aspect ratio of 1x1 for all images).

Fixes #1077

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to chose either at their option.

[fintelia]

It might be good to have the pixel density setting be part of the ImageEncoder trait that @aschampion is looking into adding for #1076

[lovasoa]

@fintelia : do you want to wait until ImageEncoder is merged, and then re-do this PR against the new trait ? Or can we merge this one first, and then let the ImageEncoder PR use that ?

[lovasoa]

Also: Do you think it would be a good idea to reuse the types defined in image-rs/jpeg-decoder#115 once it is merged ?

[aschampion]

It might be good to have the pixel density setting be part of the ImageEncoder trait that @aschampion is looking into adding for #1076

Many of the format encoders' constructors and parameters are similar, so it might eventually make sense to make some common subset of these or other settings part of ImageEncoder's constructor/builder/configuration, but for now I was going to abstract only the encode method as a minimal initial change. So one still manually constructs the format encoder they want (as in save_buffer_impl), which would not conflict with these changes. No need to wait on my PR.

[fintelia]

Other than changing the return type, I think it is worth going ahead and merging these changes. Longer term, I'd like to have more consistent interfaces for decoders (and probably merge jpeg encoding and decoding into the same crate) but we shouldn't let that delay progress now. We're still not really close to 1.0 so we are free to make breaking changes as needed

[fintelia]

I'll merge this once CI finishes

[lovasoa]

Thank you !