Evaluate estimates of image compressed size

[spillerrec]

The image gradient sum method is quite ad-hoc and doesn't really work that well. It does work though and is decently fast. Using real compression results is only very slightly better at a horrible slowdown (quality 0). But is it really better than just counting non-transparent pixels? Lets try making some different methods and properly evaluate speed / accuracy.

Suggestions to add:

• Count amount of transparent pixels
• Use LZ4 compression as suggested on FLIF gitter
• Use LZ4 compression with PNG delta compression methods