annotate plugin draw interface not working (e.g. dbus method call)

Hi,

this is a problem that I've had for some time, but finally had some time to look into this.

Basically, the draw method exposed by the annotate plugin on DBus is not working. My use case is easystroke (https://github.com/markdstjohn/easystroke), but see below a minimal example to reproduce this.

How to reproduce:

1. Enable annotate and dbus plugins.
2. Make sure annotate is working normally with the keybinding associated.
3. Form a terminal run:
dbus-send --print-reply --type=method_call --dest=org.freedesktop.compiz /org/freedesktop/compiz/annotate/screen0/draw org.freedesktop.compiz.activate string:tool

Expected behavior:
A line is drawn in the top left corner in the screen.

Actual behavior:
Nothing happens.

(Note: optional parameters x1,x2,y1,y2 could be used to specify the coordinates to draw to, but these are not necessary for this example -- it should work without giving any coordinates as well.)

Cause of bug (in plugins/annotate/src/annotate.cpp):
 -- "tool" variable in AnnoScreen::draw() gets overwritten between lines 305 and 322 (is it undefined behavior to save the c_str() of the value returned by CompOption::getStringOptionNamed()?)
 -- AnnoScreen::draw() needs to call handleEventSetEnabled() and glPaintOutputSetEnabled() to actually draw anything on the screen

I'm attaching a diff (for plugins/annotate/src/annotate.cpp) that fixes the issue for me.

Note: I've tried this on compiz 0.9.13 on Ubuntu 18.04 and I did not try on 0.9.14. Nevertheless, annotate.cpp is the same for 0.9.13 and 0.9.14, so I'd be surprised if there was a difference.