Raster paint engine renders partly garbage with source composition and opacity on formats other than RGB32/ARGB32PM

When drawing an image or with a texture or pattern brush, with source composition, opacity less than 1 and target format different from RGB32 and ARGB32_Premultiplied, some garbage pixels appear and the opacity is ignored. It happens for all transformation types and with or without SmoothPixmapTransform. The bug does not appear if antialiasing is enabled.