QRect: fix UB (int overflow) in center()

QRect::center() should be defined for any
  QRect(x1,y1,x2,x2), INT_MIN <= x1, x2, y1, y2 <= INT_MAX
because the average of two signed integers is always
representable as a signed integer.

But not when it's calculated as (x1+x2)/2, since that
expression overflows when x1 > INT_MAX - x2.

Instead of playing games with Hacker's Delight-style
expressions, or use Google's patented algorithm, which
requires two divisions, take advantage of the fact that
int is not intmax_t and perform the calculation in the
qint64 domain. The cast back to int is always well-
defined since, as mentioned, the result is always
representable in an int.

Fix a test-case that expected a nonsensical result due
to overflow.

[ChangeLog][QtCore][QRect] Fixed integer overflow in
center(). This fixes the result for some corner-cases
like a 1x1 rectangle at (INT_MIN, INT_MIN), for which
the previous implementation could return anything
(due to invoking undefined behavior), but commonly
returned (0, 0).

Change-Id: I1a885ca6dff770327dd31655c3eb473fcfeb8878
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>