Make sure cut_surface gets a neutral surface.

Also change its return to an optimised surface. Fixes bug #20876.

From the comment added to the source:
Unlike other image functions cut_surface does not convert the input
surface to a neutral surface, nor does it convert its return surface
to an optimised surface.

Since it seems to work for most cases, rather change this caller instead
of the function signature. (The issue was discovered in bug #20876).

src/image_modifications.cpp

@@ -216,7 +216,17 @@ surface crop_modification::operator()(const surface& src) const
 		ERR_DP << "start Y coordinate of CROP modification is negative - truncating to zero\n";
 		area.y = 0;
 	}
-	return cut_surface(src, area);
+
+	/*
+	 * Unlike other image functions cut_surface does not convert the input
+	 * surface to a neutral surface, nor does it convert its return surface
+	 * to an optimised surface.
+	 *
+	 * Since it seems to work for most cases, rather change this caller instead
+	 * of the function signature. (The issue was discovered in bug #20876).
+	 */
+	return create_optimized_surface(
+			cut_surface(make_neutral_surface(src), area));
 }
 
 const SDL_Rect& crop_modification::get_slice() const