lavfi: factorize drawgrid and drawbox code
lavfi/drawbox: restore verbal form for drawbox options descriptions
configure: Autodetect VA-API

This fixes a regression (reproducible since 1.2) and restores the
behaviour of 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 1.0 and 1.1.