Improve Jpeg Decoder

[antonfirsov]

Removed all my previous notes here, because my new investigations outdated them.

This point should be replaced by a WIP PR. (Continuing #274 or open a new one for a clean start.)

I'll try to put my notes here later.

[JimBobSquarePants]

Here's a list of additional issues related to jpeg that we would like to get help with.

https://github.com/JimBobSquarePants/ImageSharp/issues?q=is%3Aissue+is%3Aopen+label%3Aarea%3Aformats%3Ajpeg

[antonfirsov]

@firenero thanks for the refactor help you done here!
We still need to implement that MemoryManager to provide more control over the ArrayPool behaviour, but it's not a Jpeg-specific task to implement it.

It would be nice to find people to help with the other (harder) points or the smaller issues from this list!

[firenero]

@antonfirsov np, I'm glad to help!
I'm also going to help you with other things soon (just have a lack of time right now).

[antonfirsov]

I started to work on the generic compression stuff. See:

• PackedBuffer
• CompressedBuffer
• CompressedBufferTests

My plan is to store the JpegDecoderCore.DecodedBlocks in a CompressedBuffer when PerformanceCharacteristic == LowestMemory and use DummyPackedBuffer otherwise. The same could be done to jpeg image channels later.

[antonfirsov]

Closing this in favor of #298.