Fix alignment problem with text crescendi

Fix alignment problem with text crescendi

Text crescendi starting at skips caused a
"Programmierfehler: Improbable offset for stencil: nan staff space"
error message and moved the text crescendi to the very beginning of the
staff.
Instead of grob->extent(...), use robust_relative_extent (grob, ...), which
also works with skips.

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>