Fix [-Wtype-limits] in bundled GD lib by using signed integers

As seen in the gdImageRotateBicubicFixed() function the same setup
occurs but it uses signed integers, therefore we use also use
signed integers in gdImageRotateBilinear()

Moreover, these two functions have been removed upstream in
libgd/libgd@bd6d2e1
therefore we should also mimic upstream and remove them...

Thanks to @cmb69 for pointing it out.

Author: Girgias

ext/gd/libgd/gd_interpolation.c

@@ -1771,8 +1771,8 @@ gdImagePtr gdImageRotateBilinear(gdImagePtr src, const float degrees, const int
 			const gdFixed f_j = gd_itofx((int)j - (int)new_width/2);
 			const gdFixed f_m = gd_mulfx(f_j,f_sin) + gd_mulfx(f_i,f_cos) + f_0_5 + f_H;
 			const gdFixed f_n = gd_mulfx(f_j,f_cos) - gd_mulfx(f_i,f_sin) + f_0_5 + f_W;
-			const unsigned int m = gd_fxtoi(f_m);
-			const unsigned int n = gd_fxtoi(f_n);
+			const int m = gd_fxtoi(f_m);
+			const int n = gd_fxtoi(f_n);
 
 			if ((m >= 0) && (m < src_h - 1) && (n >= 0) && (n < src_w - 1)) {
 				const gdFixed f_f = f_m - gd_itofx(m);