Backport fixes for Weston 13

This patch series for Weston 13 contains the following fixes:

• !1398 (merged) (Desktop shell fixes)
• !1416 (merged) (libweston-desktop: Work around crash when opening popup menu)
• !1417 (merged) (ivi-shell: clear seat focus if necessary when a surface is destroyed)
• !1425 (merged) (backend-drm: fix confused fallback format handling)
• !1441 (merged) (libweston: Clip damage to paint node visibile region)
• !1442 (merged) (backends: Move damage flush into backends)
• !1443 (merged) (backend-drm: Don't force non-opaque overlays to primary plane)
• !1445 (merged) (backend-drm: Sort planes by faked zpos)
• !1447 (merged) (libweston-desktop: Fix weston crash when lost shsurf)
• !1451 (merged) (desktop-shell: Avoid using maximized size in fullscreen)
• !1489 (merged) (libweston: Add paint node destruction into weston_layer_entry_remove)

All of them applied cleanly.

@derekf has two more fixes, in !1493 (merged) which are not included here:

• !1485 (merged) (gl-renderer: apply output transform before readback in repaint)
• !1435 (merged) (Fix stride errors)