Don't hang when display objects are displaced by line or wrap-prefixes

Don't hang when display objects are displaced by line or wrap-prefixes

This fixes a hang that would frequently rear its ugly head while
displaying messages in the `telega.el' instant messenger client,
which inserts images approaching the width of the window with
line and wrap prefixes.

* src/xdisp.c (move_it_in_display_line_to): If a line or wrap
prefix is set in place, do not generate continuation lines until
a minimum of one glyph has been produced outside that prefix.
(move_it_to): Remove the previous workaround that could not
recover from errors caused by display strings.
(display_line): Synchronize with move_it_in_display_line_to;
remove old workaround that only provided for oversized wrap
prefixes comprising `space' display objects.