Fix bug #6671 with recentering and other scrolling problems.
src/xdisp.c (SCROLL_LIMIT): New macro.
(try_scrolling): Use it when setting scroll_limit. Limit
scrolling to 100 screen lines.
(redisplay_window): Even when falling back on "recentering",
position point in the window according to scroll-conservatively,
scroll-margin, and scroll-*-aggressively variables.
(try_scrolling): When point is above the window, allow searching
as far as scroll_max, or one screenful, to compute vertical
distance from PT to the scroll margin position. This prevents
try_scrolling from unnecessarily failing when
scroll-conservatively is set to a value slightly larger than the
window height. Clean up the case of PT below the margin at bottom
of window: scroll_max can no longer be INT_MAX. When aggressive
scrolling is in use, don't let point enter the opposite scroll
margin as result of the scroll.
(syms_of_xdisp) <scroll-conservatively>: Document the
threshold of 100 lines for never-recentering scrolling.
doc/emacs/display.texi (Auto Scrolling): Document the limit of 100 lines
for never-recentering scrolling with `scroll-conservatively'.