Fix PNG memory leak

[rsshilli]

This is to fix for #861. I created it on top of @blikblum's very awesome convert-es6 branch that he created for #859 so please merge that first.

[blikblum]

Some remarks:

• Can you test replacing zlib.deflate by zlib.deflateSync?
• The es6 convert branch is a moving target. Currently i'm expanding tests so can continue the conversion
• In process of removing deccafeinate artifacts i removed some code that impacts performance and also identified some low hang fruits to reduce memory usage, so when conversion is finished there should be some perf gain

[blikblum]

I created a build using deflateSync

You can test by using pdfkit-es@pngsync version

[rsshilli]

I haven't tried it and honestly the next few weeks are a big release so I doubt I'll get to it soon. But this looks like a good solution. I don't see why deflateSync wouldn't work.

[blikblum]

Probably fixed by #889 . The issue still can occur because png-js decodePixels calls zlib inflate async, but odds are much smaller.