render: Check whether the root surface isn't NULL in view_render_to_buffer

This might happen with already unmapped but not yet destroyed views.

Also, allow view_render_to_buffer to indicate failure and forward that to the client in thumbnail_frame_handle_copy.

mentioned in issue OS-issues#159

@guido.gunther ping

added 4 commits

• fcdc2394 - 1 commit from branch Librem5:master
• 32ced5ee - render: Allow view_render_to_buffer to indicate failure
• 8f4f6310 - phosh: Handle view_render_to_buffer failing in thumbnail_frame_handle_copy
• 9143b870 - render: Check whether the root surface isn't NULL in view_render_to_buffer

Compare with previous version

merged

Now with a better connection again: FWIW the changes make sense.