Sometimes old text cache entries aren't cleared to make space for new ones

When using the spark monotype font engine, in a Flickable with a lot of text items (for example using Repeater) scrolling to the end makes the UI updates become much slower.

It appears the text cache entries for the text items in the beginning of the list are always kept, so there is no space to make room for new ones when scrolling to the end, so they are always redrawn without using the text cache.