Fix bug #12811 with scrolling under scroll-up/down-aggressively.
src/xdisp.c (try_scrolling): Fix correction of aggressive-scroll amount when the scroll margins are too large. When scrolling backwards in the buffer, give up if cannot reach point or the scroll margin within a reasonable number of screen lines. Fixes point position in window under scroll-up/down-aggressively when point is positioned many lines beyond the window top/bottom.
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