XCB and 32- vs 64-bit 'long'

[Paul Eggert]

Valgrind pointed out a bug in recent XCB-related changes, in that Xlib 
converts 32-bit quantities to 'long' but XCB does not. The distinction 
matters on platforms like x86-64 where 'long' is 64 bits. Also, on my 
platform at least, xcb_get_property_value_length returns a byte count, 
not a word count; unfortunately this does not appear to be documented 
anywhere.

I just now installed a patch that I hope fixes this:

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=4c4b520520cf6b99ce7128331a4e108e58095705

Ken, could you please check this patch? I am by no means an expert in 
XCB. Thanks.

While we're on the subject, I assume that the goal is to replace all 
Xlib calls with XCB calls, on platforms that have XCB. Can Emacs start 
assuming XCB, and drop the old Xlib-only code? That would simplify Emacs 
maintenance.