[NTG-context] Let chapter start on odd page and make the even page before show an image

[Jan U. Hasecke]

Am 29.05.20 um 18:03 schrieb Aditya Mahajan:
> The magic of loading and reusing them happens behind the scenes, so the
> user doesn't have to worry about it. See
> https://wiki.contextgarden.net/System_Macros/Key_Value_Assignments#Multi-pass_data
> for more details.
> 
> Now, suppose you wanted to use the image defined as part of
> `\startchapter` as the background image of the _current_ page, then you
> could access the value of the \structureuservariable. For example,
> 
> \startchapter[title=whatever][image=filename]
> 
> \structureuservariable{image}
> 
> \stopchapter
> 
> But you want to use the value of the image _before_ the start of the
> chapter. So, Wolfgang is using the two-pass mechanism to store the value
> of the image in the database named 'chapter` in the hash-location
> `\ChapterCounter` and then retrieving it before it is defined!

Thanks a lot for the clarification!
juh