Fix misleading variable type

We port libgd/libgd@0414bb2.

Author: cmb69

ext/gd/libgd/gd_interpolation.c

@@ -1321,8 +1321,8 @@ static gdImagePtr gdImageScaleBilinearTC(gdImagePtr im, const unsigned int new_w
 			gdFixed f_j = gd_itofx(j);
 			gdFixed f_a = gd_mulfx(f_i, f_dy);
 			gdFixed f_b = gd_mulfx(f_j, f_dx);
-			const gdFixed m = gd_fxtoi(f_a);
-			const gdFixed n = gd_fxtoi(f_b);
+			const long m = gd_fxtoi(f_a);
+			const long n = gd_fxtoi(f_b);
 			gdFixed f_f = f_a - gd_itofx(m);
 			gdFixed f_g = f_b - gd_itofx(n);