Scrolling huge buffers and cc-mode

[Dmitry Antipov <dmantipov@yandex.ru>]

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Attached is a C++ source file to reproduce the painfully slow scrolling
under cc-mode. Test is simple: just visit it and press <page-down>, or <next>,
or whatever else bind to scroll-up-command, until buffer end is reached.
The file is ~900K; when scrolling reaches 50%, everything is stopped, may
be up to a few minutes (!), there is no response to any keyboard input
including C-g, and CPU consumption is ~80-90%. After that, scrolling is
resumed, but it's still _very_ slow. This looks like a C++ issue only
since xdisp.c of nearly the same size may be scrolled much faster.

Dmitry

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