Streaming API improvements

[randy408]

The streaming API is similar to libpng's which is great when it comes porting code but it has some drawbacks, this issue is for listing potential improvements.

[randy408]

The current streaming API expects the read callback to copy data to a library-controlled buffer, this introduces unnecessary copying if the user is also storing the PNG in memory.

Add a new function spng_set_png_stream2() that takes a different type of callback:

typedef int spng_read_fn2(spng_ctx *ctx, void *user, void **dest, size_t length);

This callback function assigns the pointer dest to the requested amount of bytes.

[michaelforney]

It also might be nice to allow a push-style or asynchronous decoding interface. For example, in netsurf's png handler, it receives image data in chunks, and pushes it into libpng with png_process_data:

http://git.netsurf-browser.org/netsurf.git/tree/content/handlers/image/png.c#n315

I'm don't believe there is a way to do this with the current libspng API.

[randy408]

I created a new issue for that: https://github.com/randy408/libspng/issues/39