LibGfx: Free the harfbuzz buffer when measuring text width

I spotted this leak when WebContent was exiting with ASan enabled on a
page with a media element. MediaPaintable calls Gfx::Font::width(),
which calls through to measure_text_width(), which then drops an
hb_buffer_t* without freeing it.

Libraries/LibGfx/TextLayout.cpp

@@ -191,6 +191,7 @@ float measure_text_width(Utf16View const& string, Font const& font, ShapeFeature
     for (size_t i = 0; i < glyph_count; ++i)
         point_x += positions[i].x_advance;
 
+    hb_buffer_destroy(buffer);
     return point_x / text_shaping_resolution;
 }