weston_surface::keep_buffer should be replaced with proper reference counting

Quoteth @daniels:

[...] in future we should probably make either the renderer or the backend take a ref on the buffer and just always drop it. keep_buffer has always felt like a horrible layering violation ... not least because it's a boolean when you can have multiple views, multiple outputs, multiple backends, etc.

!578 (comment 1967089) for context.