Protect bidi cache from inadvertent resets

Protect bidi cache from inadvertent resets

* src/xdisp.c (Fline_pixel_height, Fmove_point_visually): Save and
restore the bidi cache, to avoid inadvertently resetting it by
starting a new iteration through buffer text.  This could cause
trouble if these functions are called during a redisplay cycle,
especially while we were processing RTL text.