ping/progress callbacks

[jmbnyc]

I want to ask if you have any plans to add a ping/progress API for compress/decompress. I am using your library in a system where some operations can take multiple seconds. All of our threads are being monitored and an inability to "ping" the monitor (just a simple atomic increment) can cause false alarms of potentially stuck threads. I added a mechanism for my use and it works well. However, I wanted to check if this is something you would consider adding.

[jmbnyc]

I should mention that I made 2 additions to the library. I added the ping callback for both compress and decompress. In addition I added an decompress initial interval callback plus atomic updating of a inflated byte count that is passed from the use (uint64_t*). The reason for this is simple: in my case I want inflation from a "producer thread" and processing in an "consumer" thread. The initial interval callback allows the consumer to start once the initial data is available (my code just signals a futex). I then process the data up to the inflated byte count which is being updated in during decompress. I opted for the atomic update of the passed pointer to uint64_t over a callback because I wanted to add as little overhead as possible esp because a callback would just increment the inflated byte count value, i.e. no need for the callback call overhead.

[pavetheway91]

Also interested in this in order to have a progress spinner to look at, while libdeflate is compressing.

Could you open a PR with your changes?