Rendering text *into* an image

[Joseph Garvin]

Is there a way in emacs lisp to render text *into* an image? To be clear, I
don't mean I want to display an image in an emacs buffer. I mean I want to
have emacs render some text as if it was going to be displayed but instead
of displaying it to save it into an image file. I've found (create-image)
for working with already existing images and byte arrays but I haven't
found any way to get emacs to render text into a new one.

My goal is to render characters with diacritical marks (umlauts, accents,
etc.) where the character and the mark are different colors -- emacs will
not normally do this, if you try propertizing a character and a mark on
that character differently the character just wins and it's all the same
color. So given a character and a diacritical mark I want to render an
image with the character and diacritical mark in one color, then render
over it again without the mark in a different color (pictures of this
technique here: http://tex.stackexchange.com/a/97224), then use an overlay
with the display property to display it instead of the original character.

I could generate the image outside emacs but then the rendering engine
won't be the same and the result might look inconsistent with the rest of
the buffer.