Dangerous cast between cairo_image_surface_t and cairo_image_source_t

I'm writting a cairo backend to use some hw acceleration. Basically I'm
overloaded cairo_image_surface_t and cairo_image_source_t.
I'm facing a crash in one of my functions and this is because of a cast
from cairo_image_surface_t to cairo_image_source_t.

In cairo-span-compositor.c:composite_aligned_boxes() :

if need_clip_mask is true it creates a mask using get_clip_surface() that
returns a cairo_image_surface_t*
Then if no_mask is true it will call compositor->composite_boxes and gives
a  cairo_image_surface_t* mask as parameter instead of
a cairo_image_source_t*


*Conditions:*
*no_mask = 1*
*need_clip_mask = 1*

if (need_clip_mask) {
   * mask = get_clip_surface* (compositor, dst, extents->clip,
     &extents->bounded);
    if (unlikely (mask->status))
return mask->status;

    mask_x = -extents->bounded.x;
    mask_y = -extents->bounded.y;
}

if (! no_mask) {

...
}

src = compositor->pattern_to_surface (dst, source, FALSE,
      &extents->bounded,
      &extents->source_sample_area,
      &src_x, &src_y);
if (likely (src->status == CAIRO_STATUS_SUCCESS)) {
    status = *compositor->composite_boxes* (dst, op, src, *mask*,
  src_x, src_y,
  mask_x, mask_y,
  0, 0,
  boxes, &extents->bounded);


I welcome your thoughts,

Yannick

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