Reimplement `backtrace-on-redisplay-error` using `push_handler_bind`.
This moves the code from `signal_or_quit` to `xdisp.c` and
`debug-early.el`.

* lisp/emacs-lisp/debug-early.el (debug-early-backtrace):
Add `base` arg to strip "internal" frames.
(debug--early): New function, extracted from `debug-early`.
(debug-early, debug-early--handler): Use it.
(debug-early--muted): New function, extracted (translated) from
`signal_or_quit`; trim the buffer to a max of 10 backtraces.

* src/xdisp.c (funcall_with_backtraces): New function.
(dsafe_calln): Use it.
(syms_of_xdisp): Defsym `Qdebug_early__muted`.

* src/eval.c (redisplay_deep_handler): Delete var.
(init_eval, internal_condition_case_n): Don't set it any more.
(backtrace_yet): Delete var.
(signal_or_quit): Remove special case for `backtrace_on_redisplay_error`.
* src/keyboard.c (command_loop_1): Don't set `backtrace_yet` any more.
* src/lisp.h (backtrace_yet): Don't declare.

Make sure we build the (ERROR-SYMBOL . ERROR-DATA) object only once
when signaling an error, so that its `eq` identity can be used.
It also gets us a tiny bit closer to having real "error objects"
like in most other current programming languages.

* src/eval.c (maybe_call_debugger): Change arglist to receive the error
object instead of receiving the signal and the data separately.
(signal_or_quit): Build the error object right at the beginning so it
stays `eq` to itself.
Rename the `keyboard_quit` arg to `continuable` so say what it does
rather than what it's used for.
(signal_quit_p): Change arg to be the error object rather than just the
error-symbol.

* src/keyboard.c (cmd_error_internal, menu_item_eval_property_1):
Adjust calls to `signal_quit_p` accordingly.

* test/src/eval-tests.el (eval-tests--error-id): New test.

Rather than blindly increase `max-lisp-eval-depth` when entering the
debugger or running `signal-hook-function`, use this new "reserve"
to keep track of how much we have grown the stack for "debugger"
purposes so that for example recursive calls to `signal-hook-function`
can't eat up the whole C stack.

* src/eval.c (max_ensure_room): Rewrite.
(restore_stack_limits): Move before `max_ensure_room`.  Rewrite.
(call_debugger, signal_or_quit): Adjust calls accordingly.
Also grow `max-lisp-eval-depth` for `hander-bind` handlers.
(init_eval_once): Don't initialize `max_lisp_eval_depth` here.
(syms_of_eval): Initialize it here instead.
Add new var `lisp-eval-depth-reserve`.

* doc/lispref/eval.texi (Eval): Add `lisp-eval-depth-reserve`.

Move ad-hoc code meant to ease debugging of bootstrap (and batch mode)
to `top_level_2` so it doesn't pollute `signal_or_quit`.

* src/lisp.h (pop_handler, push_handler_bind): Declare.
* src/keyboard.c (top_level_2): Setup an error handler to call
`debug-early` when noninteractive.
* src/eval.c (pop_handler): Not static any more.
(signal_or_quit): Remove special case for noninteractive use.
(push_handler_bind): New function, extracted from `Fhandler_bind_1`.
(Fhandler_bind_1): Use it.
(syms_of_eval): Declare `Qdebug_early__handler`.
* lisp/emacs-lisp/debug-early.el (debug-early-backtrace): Weed out
frames below `debug-early`.
(debug-early--handler): New function.

AFAIK, this provides the same semantics as Common Lisp's `handler-bind`,
modulo the differences about how error objects and conditions are
represented.

* lisp/subr.el (handler-bind): New macro.

* src/eval.c (pop_handler): New function.
(Fhandler_Bind_1): New function.
(signal_or_quit): Handle new handlertypes `HANDLER` and `SKIP_CONDITIONS`.
(find_handler_clause): Simplify.
(syms_of_eval): Defsubr `Fhandler_bind_1`.

* doc/lispref/control.texi (Handling Errors): Add `handler-bind`.

* test/src/eval-tests.el (eval-tests--handler-bind): New test.

* lisp/emacs-lisp/lisp-mode.el (lisp-font-lock-keywords):
Move 'handler-bind' from CL-only to generic Lisp.
(handler-bind): Remove indentation setting, it now lives in the macro
definition.

* src/sfnt.h (SFNT_ROUND_FIXED): New macro.

* src/sfntfont.c (sfntfont_get_glyph_outline): Don't apply
advance width distortion before the glyph is instructed or
decomposed.  Round advance width as measured between both
phantom points subsequent to instruction code execution.
(sfntfont_draw): Don't take the advance's ceiling when advancing
origin point.

* src/sfnt.c (sfnt_scale_metrics):

* src/sfntfont.c (sfntfont_get_glyph_outline): Round advance and
floor lbearing scaling glyph metrics.
(sfntfont_measure_pcm): Don't round or truncate metrics which
have already been.

* src/sfnt.c (sfnt_fedge_sort): Delete function.
(sfnt_poly_edges_exact): Don't sort edges, iterate through each
instead.
(main): Adjust tests.