Ways to control PNG and JPEG encoding

[p0nce]

Basically quality is fixed right now. Need to see how FreeImage did that in its API.

[p0nce]

Basic design in #78

[p0nce]

• What is the fastest PNG filter? Take that for ENCODE_PNG_FILTER_FAST
• Do similar thing for JPEG compressor, since it can do 4:2:0 and 4:4:4, a most interesting parameter