gsk: Fix a maybe-uninitialized warning

The compiler (gcc 13.2) thinks that `t` could be used uninitialised.
That’s obviously not the case, because there’s always going to be at
least one loop iteration due to the initial values of `t1` and `t2`.

Change the loop to a `do…while` to make that a bit clearer to the
compiler without making any functional changes to the code.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gsk/gskcurve.c

@@ -227,7 +227,7 @@ get_t_by_bisection (const GskCurve *curve,
   t1 = 0;
   t2 = 1;
 
-  while (t1 < t2)
+  do
     {
       t = (t1 + t2) / 2;
       if (t == t1 || t == t2)
@@ -243,6 +243,7 @@ get_t_by_bisection (const GskCurve *curve,
       else
         t2 = t;
     }
+  while (t1 < t2);
 
   return t;
 }