devdraw, acme, snarfer: remove 64KB snarf buffer limit

The snarf code silently dropped text larger than SnarfSize (64KB),
causing paste to produce nothing. Replace the static clip buffer
with a dynamically allocated one, remove all the size checks, and
read the actual X11 property size instead of capping at SnarfSize.

src/cmd/devdraw/mac-screen.m

@@ -1109,7 +1109,7 @@ rpc_getsnarf(void)
 void
 rpc_putsnarf(char *s)
 {
-	if(s == nil || strlen(s) >= SnarfSize)
+	if(s == nil)
 		return;
 
 	dispatch_sync(dispatch_get_main_queue(), ^(void) {