sourcepos broken for tables and inlines and confusing CMARK_OPT_SOURCEPOS behavior

[martincizek]

We've implemented an automated GFM editor, which depends heavily on sourcepos, which is currently widely broken. I will sum app all the issues here and then will link a pull request with the fixes.

I wanted to wait until the overlapping upstream pull request commonmark#298 is merged, but nothing happens for more than a year, although the objections against performance turned out to be negligible.

I kindly ask you to review this, as the GfmEditor is one of exciting use cases enabled by sourcepos. That means, incorrect sourcepos is not just a cosmetic bug affecting rendering appearance. It prevents whole category of automatic processing that would be possible with it.

1. Sourcepos is incorrect on tables with preceding table-like paragraph:

   | paragraph |
   | header |
   | --- |
   | row |
   

2. CMARK_OPT_SOURCEPOS is very confusing in terms how it affects the parser:
   • when it is not set on the parser, sourcepos is currently broken on all inlines
   • when it is set on the parser, sourcepos gets improved, but it is still far from correct
3. Sourcepos is broken on strikethrough with softbreak:

   Hi ~~friend~~ and ~~other
   friend~~.
   

4. Sourcepos is missing on softbreaks and linebrekas (at least we missed it a lot :))
5. Sourcepos is broken for things mentioned in upstream PR Fix source positions for inlines. commonmark/cmark#298, i.e.:
   • Inlines inside inconsistently indented blocks.
   • Links/images.
   • Autolinks.
   • Escaped newlines.
   • Non-matched backticks.