Need to think a bit more about locking,
but overall it's a working devdraw.

Multiclient mode may not be working
but nothing is using it yet.

Also turn mac-srv.c into a generic srv.c,
so we can remove the duplication with x11-srv.c.

Now that we only have Metal, we can drop the -metal.
Also now that Carbon is gone we can drop the macargv.c,
and then the -objc from object file names.

This replaces the pixel-art scaling algorithm used for upscaling before.
The results were not crisp enough to serve as everyday cursors.

Add a new macOS cocoa screen, cocoa-screen-metal.m.
Rewrite the macOS cocoa drawing code to use the builtin runloop,
and use Metal to push pixels with CAMetalLayer.

Remove all of the deprecated code, and simplify some of the logic.
Modify mkwsysrules.sh such that the new code is used only when
the system version is equal or higher than 10.14.

Allow touch events to simulate mouse clicks:
three finger tap for the middle mouse button;
four finger tap for the 2-1 chord.

Support Tresize.

Scale 16x16 Cursor up to 32x32 with an EPX algorithm.

Support macOS input sources including the basic dead keys and the
advanced CJK input methods.

Increase the communication buffers in cocoa-srv.c to allow more
input, especially for long sentences prepared by the macOS input
souces.

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/5528046

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/5015042