free Material 'shadow_material_double_sided' on ::finish()

(cherry picked from commit 7a9998370f)

drivers/gles2/rasterizer_gles2.cpp

@@ -10998,6 +10998,7 @@ void RasterizerGLES2::finish() {
 
 	free(default_material);
 	free(shadow_material);
+	free(shadow_material_double_sided);
 	free(canvas_shadow_blur);
 	free( overdraw_material );
 }