xwayland-eglstream: fix X11 rendering to flipping GL / VK window

If a window is being used for direct rendering with OpenGL or Vulkan, and is using the flipping path for presentation, it's pixmap will be set to a dma-buf backed pixmap created by the client-side GL driver. However, this means that xwl_glamor_eglstream_post_damage won't work since it requires that the pixmap has an EGLSurface that it can render to, which dma-buf backed pixmaps do not.

In this case, though, xwl_glamor_eglstream_post_damage is not necessary since glamor will have rendered directly to the pixmap, so we can simply pass it directly to the compositor. There's no need for the intermediate copy we normally do in that function.

Therefore, this change adds an early-return case to post_damage for dma-buf backed pixmaps, and removes the corresponding asserts from that function and allow_commits.