We're storing in the texture cache images and scaled images appending
the scaling factor to the key. When a file changes the cache key
corresponding to that file is removed, but not the keys for the scaled
ones so that images in the cache are never reloaded.

This patch removes all keys from the cache related to the file that
changes, including those with the scaling factor.

A new set (hash table) was added to keep track of scale used to be able
to remove all possible images in the cache.

When the KEY is removed from the cache, we can look now in the scale set
for and each scale we also remove the key "KEY1.000000", "KEY2.000000",
etc.

Assuming that the number of used scales is small (I would typically
expect one or two), the overhead should be negligible.

GNOME/gnome-shell!567