Fix bug #6671 with point position in the window when scrolling far away.

Fix bug #6671 with point position in the window when scrolling far away.

 src/xdisp.c (redisplay_window): Don't check buffer's clip_changed
 flag as a prerequisite for invoking try_scrolling.
 (try_scrolling): Limit scrolling to 100 screen lines.
 (SCROLL_LIMIT): New macro.
 (try_scrolling): Use it when setting scroll_limit.
 (redisplay_window): Even when falling back on "recentering",
 position point in the window according to scroll-conservatively,
 scroll-margin, and scroll-*-aggressively variables.