[Cin] Add in manual

[Andrea paz]

I attach an addition for the manual (Overlay chapter) regarding
problems due to handling transparencies. I am attaching both Latex and
txt versions if you would like to make corrections/improvements; but
even before that is to decide whether to use this text or not. There
has been a lot of talk lately about the size achieved by the manual,
and this addition goes beyond the request for a simple description of
CinGG's functionality, and is a more theoretical and discursive part.
It is up to you to decide whether it should be put in or is better
not.

The texts in square brackets are about the new Blend Algebra and Blend
Program plugins, so they are to be ignored until it is decided whether
to implement the plugins or not. If the plugins are integrated, I will
go ahead and synthesize Georgy's explanations for the manual entry.
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