doc: Improved processing of SHD and cvAuto colors in CHPs.

* There was a bug in SHD interpretation so a few solid shadings were missed.
* Changed the order of properties processing in case of named styles to:
PAP -> CHP to improve automatic colors support.
* The same order should apply for text chunks, but that's a bit complicated
at the moment so the background-color from PAP or parent's PAP is obtained
before any chunks are processed.

TODO:
> Use the background-color from CHP of a named text style if applicable
on the text chunk level.