[cairo] Bug drawing PNG images with transparency

[Franz Schmid]

Franz Schmid schrieb:

>> For starters, which backend are you using?
> 
> I'm using the image backend.
> 
Made some more tests with Scribus and other small test
applications on that issue. It comes out that the
image surface is responsible for the vanishing transparency,
if i use the xlib backend everything works as expected.

Sadly i can't use the xlib backend in Scribus as i need to
get access to the image data for implementing our special
blendmodes.

Drawing an image with transparent areas onto a cairo_image_surface
fails with the transparency information lost in the following cases:
1) a cairo_scale operator != 1 is applied to the surface
2) the image is painted with cairo_paint_with_alpha with an alpha
    value != 1, in this case the former transparent area has some
    weired color shifts
3) a cairo_rotate operation is applied to the surface
4) a clipping paths is set

I hope that these informations help debugging that a bit.

Greetings,
Franz Schmid