usp10: Do not fall back to presentation form B if the contextual feature exists in ContextualShape_Arabic().

E.g., the base form of 'ا' may be the same as the isolated form, in which case
the "isol" feature may not have a substitution for it. At the same time, while
the presentation form B glyph may look the same or similar as the base form,
it may have a different glyph ID. This can break e.g.
GSUB_LOOKUP_CONTEXT_CHAINED substitutions because of the unexpected glyph ID.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Aric Stewart <aric@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>