16-bit PNG and raw image support

[perlman]

I am reviving a long overdue thread for better png support (#76).

As of now, this branch adds a "png16-image" target, as well as "raw-image" (RGBA) and "raw16-image) (short gray), which I'm using for small experiment at the moment.

@trautmane: Instead of creating end-points for 16-bit, do you think the data type should be specified as an argument, e.g. &dtype=RGBA or &grayscale=true&depth=16 or something like that?

[trautmane]

Hi Eric, Apologies for not looking at this sooner. Your distinct end-points vs. query parameters question is one that I have struggled with for a while. My original (possibly now outdated) rationale for creating distinct end-points for the image APIs was to ensure they were cache friendly. Years ago some caches ignored query parameters. Even today, I think most caches are sensitive to query parameter ordering (e.g. ...grayscale=true&depth=16... and ...depth=16&grayscale=true... reference the same image but get cached separately). From a code standpoint, I think the use of query parameters is much more intuitive and maintainable. Every time we introduce another image variant, I am enticed to move to the query parameter style. Since you've already gone through the (mild) pain of adding distinct end-points here, let's keep what you've done. I'm happy to consider/discuss alternatives if you have other thoughts on the subject.

[trautmane]

Eric - I merged upstream changes and fixed one typo on a local copy of your branch. Can you allow me to commit these changes to your repo ( see https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork ) so that I can easily merge this pull request with my fixes?

[perlman]

"Allow edits from maintainers." is already checked.

These changes would be immediately useful for me to PR 16-bit support to neuroglancer, so, thanks! I will plan on adding arguments to reduce the number of endpoint permutations. The png & raw endpoints should be able to handle all byte, short, RGB and RGBA with a small amount of work.