[Groff] Groff general image feature requirement specification

[Egil Kvaleberg]

Having made a proposal for gpic, here are some thoughts on
how a generic image specification may look in groff:

1. It should be an escape sequence, so that it can be placed
"anywhere".

2. As well as being usable by gpic and in macros, the
escape sequence must also be suitable for use in strings (or directly in
a text for that matter). This will
allow small images to be used as user-defined glyphs/characters.

3. The current point should be the lower left corner of the
image. This for compatibility with the existing PostScipt
image import, but also to facilitate using tiny images as
characters/glyphs, where they should by default rise from the 
baseline.

4. Due to 2., the escape sequence must have the right width,
so that it will do the right thing within a \w'', for instance.

5. The escape sequence should produce a horizontal movement
equal to its width, but no vertical movement.

Egil
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