Without this change, the transparent objects composed over the opaque
background creates the transparency of the entire window through which
the desktop wallpaper could be seen.
This problem manifested itself with glow backend and wayland compositor
(gnome).
This patch completely eliminates this problem. With it, the final image
produced by glow backend corresponds to the image produced by the wgpu
backend.
Fixes#1018
wgpu would currently ignore the alpha: https://github.com/gfx-rs/wgpu/issues/687
glow (and naively patched wgpu) requires premultiplied alpha, so if you don't
multiply the RGB by the A right now, the semi-transparent color would be wrong
(too bright).
winit with_transparent doesn't seem necessary.
