xterm.c: Move GTK3-specific variables into narrower scopes

xterm.c: Move GTK3-specific variables into narrower scopes

Avoid declaring GTK3-specific variables in larger-than-necessary scopes,
especially under #ifdefs.  Instead, declare them where used.  This improves
readability and reduces #ifdef clutter without changing behavior.

* src/xterm.c: [HAVE_GTK3] (x_update_opaque_region):
(XTframe_up_to_date, x_new_focus_frame, handle_one_xevent)
(x_ignore_errors_for_next_request, x_stop_ignoring_errors): Clean up
variable declarations.

(cherry picked from commit 177accc53d7ed2afc5768ae117fc6a19749e8fb6)