libweston: Extend weston_surface_copy_content to aid RDP RAIL
Until now, weston_surface_copy_content() has been mostly just a debugging API, with an austere feature set.
The RDP backend intends to add support for RDP-RAIL, which allows remoting of individual client windows (rootless operation). It's useful for that backend to use this API to fetch surface content.
However, some new features (stride, y-flip, argb/abgr conversion and scaling) are necessary to suit RAIL's usage.