add shading(type 1 to type 7) rendering/create support

[anyongjin]

1. add shading(type 1 to type 7) rendering/create support
2. add unit test for shading 2,3,4
3. optimize performance while image decoding, soft masking. 'from8bit' is about 10x faster, applyMask is 2x faster.
4. fix conflicts

related issue 384

I've ’tested more than a dozen files and solved three bugs. But there may be still some bugs and need more test. So please merge this commit to a separate branch, and release it after full testing.

Thank you.

[TomRoush]

@anyongjin Wanted to give you an update on this PR. I've looked through everything you've written and it all looks great. I made some formatting changes and rebased to get it up to date with master. In comparing the rendering results of all the test PDFs, I did notice some regressions (and some bugs that were fixed). I'm going to work on debugging the issues and maybe split some of the changes into different PRs so they can be included in 2.0.24.0 even if all the changes aren't ready

[anyongjin]

Thank you for your excellent work ! you are really amazing (๑•̀ㅂ•́)و✧