Reduced overall SizeRequestCache size

Reduced overall SizeRequestCache size

This patch changes the 'age' counting previous approach taken
verbatim from clutter and changes it for a counter of validated
caches and an index to the last cached value which we use to
round-robin through the cache if ever a widget is requested
for more than 3 contextual sizes (cache is reduced by 3 * sizeof (int)
per widget private data).