SDL_LowerSoftStretchLinear: assign result from scale_mat() to ret.

otherwise it would always return -1 when SSE and NEON instrinsics
are absent.

src/video/SDL_stretch.c

@@ -850,7 +850,7 @@ SDL_LowerSoftStretchLinear(SDL_Surface *s, const SDL_Rect *srcrect,
 #endif
 
     if (ret == -1) {
-        scale_mat(src, src_w, src_h, src_pitch, dst, dst_w, dst_h, dst_pitch);
+        ret = scale_mat(src, src_w, src_h, src_pitch, dst, dst_w, dst_h, dst_pitch);
     }
 
     return ret;