Hi, it's possible to render on a video file the texture drawn into a
geo object, but the quality is always poor whatever codec we use (even
uncompressed files), that can be useful for having poor animated
textures, but anyway the quality is never as good as we can get when we
render into picture files (isn't it?).

The contents of the framebuffer are exactly what you see onscreen. I don't understand what is 'poor' about the image being read back.

We can render framebuffer into picture files with the help of
[pix_write], but I don't know any simple solution yet to render directly
into pict files what we can get with the 'unstable' [pix_record].

pix_record on OSX is extremely well tested. I've recorded in excess of one million files with it.

So my question that will certainly stay unanswered is/

What about adding a simple option to [pix_write] for rendering into
picture files only the texture created by a gemchain instead of the
framebuffer?

pix_write should just write the pix buffer to a file and not also have pix_snap functionality embedded in it. Unfortunately doing this would break a lot of patches. Maybe pix_image_write would be an acceptable option.