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Setting backtrace-on-redisplay-error to non-nil enables the generation of a
Lisp backtrace in buffer *Redisplay-trace* following an error in Lisp called
from redisplay.

* doc/lispref/debugging.texi (Debugging Redisplay): New subsection.
(Error Debugging): Reference to the new subsection.

* etc/NEWS: New entry for the new facility.

* src/eval.c (redisplay_deep_handler): New variable.
(init_eval): Initialize redisplay_deep_handler.
(call_debugger): Don't throw to top-level after calling debug-early
(internal_condition_case_n): "Bind" redisplay_deep_handler to the current
handler.
(backtrace_yet): New boolean variable.
(signal_or_quit): New code section to handle Lisp errors occurring in
redisplay.
(syms_of_eval): New DEFVAR_BOOL backtrace-on-redisplay-error.

* src/keyboard.c (command_loop_1): Set backtrace_yet to false each time around
the loop.
(safe_run_hooks_error): Allow args to be up to four Lisp_Objects long.
(safe_run_hooks_2): New function.

* src/lisp.h (top level): declare as externs backtrace_yet and
safe_run_hooks_2.

* src/xdisp.c (run_window_scroll_functions): Replace a call to
run_hook_with_args_2 with one to safe_run_hooks_2.