screen: Fix screen equality comparison (FS#1151)
We did some black magic which broke and was replaced with more black magic. This
now broke using screen objects as table indexes:
$ echo 'local l, s = {}, screen[1] ; l[s] = 42 ; return l[s]' | awesome-client
double 42
$ echo 'local l, s = {}, screen[1] ; l[s] = 42 ; return l[screen[1]]' | awesome-client
<no output>
Fix this by using just a single lua userdata for representing a screen object.
It would be even better if screens were allocated with lua, but that doesn't
really provide any benefits right now and would be more complicated...
Signed-off-by: Uli Schlachter <psychon@znc.in>