devdraw: restore compilation on OS X 10.6

Also add some ignored files to .hgignore

R=rsc
http://codereview.appspot.com/6842089

src/cmd/devdraw/cocoa-screen.m

@@ -1408,9 +1408,11 @@ winsizepoints()
 static NSSize
 winsizepixels()
 {
+#if DOSX_VERSION >= 100700
 	if (OSX_VERSION >= 100700 && devdrawretina)
 		return [win.content convertSizeToBacking: winsizepoints()];
 	else
+#endif
 		return winsizepoints();
 }