Improve the quality of text when the scale it is shown at screen is

Improve the quality of text when the scale it is shown at screen is
smaller than the original, seems to work well down to about a scale
of 50%.
* clutter/pango/pangoclutter-fontmap.c:
(pango_clutter_font_map_default_substitute): turn off hinting.
* clutter/pango/pangoclutter-render.c: request linear filtering
instead of nearest neighbour when scaling down.
* tests/test-text.c: replaced test with a a test that renders a sample
grid with various pixel sizes and scales for visual inspection of
text rendering quality.