scenes/opengl: Avoid blending for entirely opaque SurfaceItems
Blending is quite expensive especially with software rendering. In the case of Firefox on Wayland, it uses a ARGB8888 buffer but marks the entire surface as opaque, so the alpha channel can be ignored.
Tested only in a Wayland session, couldn't find anything broken. The CPU usage
while playing a video in FF went down by ~10% and apitrace shows that
GL_BLEND is disabled.