Add a way to force certain features

This is handy for debugging the rendering of featured tiles.
To enable this, compile with CFLAGS=-DDEBUG, and then set
the GNOME_SOFTWARE_FEATURED environment variable to the id
of the app you want to see featured.
Move featured image rendering to css

We are constructing custom css for the featured tile anyway,
and using css for the image lets us use full-size images, and
rely on css to do the scaling and positioning, instead of manually
clipping the image.
Better featured data for darktable and mypaint

Replace the manually cut-to-size images with full renderings
of these images, and add sizing and positioning parameters
to make them come out ok in the tile.