devdraw: MacBook retina support

Enable with export devdrawretina=1 (everything will be smaller).

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/6592072

man/man1/devdraw.1

@@ -10,6 +10,13 @@ invoked via
 .I Devdraw
 serves a custom graphics protocol and is the only program
 that talks directly to X window servers.
+On Macintosh, setting
+.BI devdrawretina
+to
+.BI 1
+will cause
+.I devdraw
+to use all available physical pixels on a retina display.
 .SH SOURCE
 .B \*9/src/cmd/devdraw
 .SH "SEE ALSO