Emacs NS 'scratch/ns/draw-to-bitmap' branch performance

[Daniel Pittman <slippycheeze@google.com>]

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G'day Alan, et al.

After building Emacs with the ns/draw-to-bitmap branch merged, on macOS
10.15.2, I'm seeing some performance issues.  I'd like, Alan, to know how
you would prefer me to proceed in a way that best helps you (and anyone
else here) resolve them.

This is on a MacBook Pro (15-inch, 2019), Intel UHD Graphics 630 1536 MB +
Radeon Pro 555X machine, driving the 2880 x 1800 internal LCD at the
default 1680 x 1050 apparent resolution.  `frame-edges` are at 78, 45 and
1677, 1049, as far as Emacs is concerned.

The trivial reproduction, for me, is to hold any key (eg: "n") to get
auto-repeat inserting it.  On this machine I see approximately 50
characters every 10 seconds, or ~ 5 "frames" per second.  This is, of
course, substantially lower than anywhere else in the OS, and also far
below what I'd experience prior to that merge.

This is a pretty clear "graphics update" cost:
(list (progn (goto-char 0) (benchmark-run 40 (next-line))) (progn
(goto-char 0) (benchmark-run 40 (next-line) (redisplay))))
=> ((0.006762000000000001 0 0.0) (9.116147 0 0.0))

I can dig into this myself, and see if I can't figure out what is slowing
everything down, but I figured I'd ask first.

PS: I didn't check yet, but one of my first points of call is going to be
the "canDrawConcurrently" flag on the NSView, since the bitmap rep means it
doesn't need to block anything while redrawing, yes?

https://developer.apple.com/documentation/appkit/nsview/1483425-candrawconcurrently?language=objc

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