If an application provides its window icon via wmhints, then mutter
loads the pixmap specified by the application into a cairo xlib surface. When
creating the surface it specifies the visual, indirectly, via an XRender
picture format.
This is suboptimal, since XRender picture formats don't have a way to specify
16bpp depth, which an application may be using.
In particular, applications are likely to use 16bpp depth pixmaps for their
icons, if the video card offers a 16bpp framebuffer/root window.
This commit drops the XRender middleman, and just tells cairo a visual to use
directly.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/715
556e7694de