Created attachment 18451 [details]
testing sample code

The following sequence produces wrong masked color:

	cairo_t* cr = cairo_create(target);

	cairo_set_source_surface(cr, source, 0, 0);
	cairo_paint_with_alpha(cr, alpha);

	cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, alpha);
	cairo_mask_surface(cr, mask, 0, 0);

where target, source, and mask are image surfaces, and alpha is an arbitrary value other than 1.0.
For example, when data is as follows:

	uchar sourceWhite[] = { 0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff };
	uchar maskA8[] = { 0x00,0xff,0x00,0x00 };
	double alpha = 0.5;

we will get this result in target surface: 80808080404040C0 while the expected should be: 80808080C0C0C0C0.


Please see the attached for a sample code.
(compile command was: g++ -Wall -ffast-math -g `pkg-config --cflags --libs cairo` cairo_mask.cpp
 g++ version: g++ (GCC) 4.2.3 (Ubuntu 4.2.3-2ubuntu7))