[cairo] RBG subpixel rendering on a SDL surface?

[yuuki]

Hi, I'm trying to use Cairo to draw on a SDL surface (i.e. an image 
surfaces).
Following this page (https://www.cairographics.org/SDL/), drawing
works fine but I can't get subpixel rendering working for fonts.

 From the doc it seems the only thing to do is to set a 
cairo_font_options_t
with the correct options to the context, but that doesn't seem to work. 
I can get
grayscale antialiasing but never RGB.

I've seen that surfaces also have a cairo_font_options_t but that this 
one cannot
be changed. So my questions are:

- Is it supposed to work or not ?
- What factors determines how fonts get drawn (context, surface, other 
things... ) ?
- Is there a way to get RBG subpixel rendering working with SDL ?


Thanks!