Speed up scrolling when lines are very long and truncated

Speed up scrolling when lines are very long and truncated

* src/window.c (window_scroll_for_long_lines): New function.
(window_scroll): Call 'window_scroll_for_long_lines' when lines
are very long and truncated on display.  Also, disable
'fontification-functions' during scrolling in that case.
* src/xdisp.c (redisplay_window): When recentering the window's
display, go back to the centering position using a simplified
method, if lines in the buffer are very long and truncated on
display.