[cairo] bug in backend to device transformation

[Jonathan Gramain]

Hello everyone

I think I've found a bug in cairo-gstate.c, concerning the matrix
transformation in the method _cairo_gstate_backend_to_user_rectangle, here
is the current code :

   cairo_matrix_t matrix_inverse;

    cairo_matrix_multiply (&matrix_inverse, &gstate->ctm_inverse,
                           &gstate->target->device_transform_inverse);
    _cairo_matrix_transform_bounding_box (&matrix_inverse,
                                          x1, y1, x2, y2, is_tight);


Shouldn't we first apply the device inverse transform before applying the
CTM inverse? I'm doing some work on a canvas based on libpapyrus, and I
encountered a problem when I tried to draw on a GdkWindow (using
gdk_cairo_create) embedded in a GtkScrolledWindow, and at the same time
applying rotations and scaling on the CTM : the scrolled window moves the
GdkWindow, thus the device transform is a translation on this window, and so
the clipping region was not right, and eventually went outside of the
visible region.


Hope this helps.

Jonathan Gramain
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