Fix an overflow bug in the text measurement cache

[nicbarker]

To address #43 🏄‍♂️
I simply forgot to implement a mechanism for clearing stale items from the text measurement cache 🤦
This PR introduces a "generation" mechanism for clearing items from the text measurement cache. The TL;DR is:

• A generation integer is assigned to text measurement cache items every time they are accessed
• When walking the linked lists of hash map items, any items passed over that are more than a few generations old will be ejected.
• If the measurement cache fills up completely (possible in some circumstances where hashes are very evenly distributed, a TODO here is to figure out ideal bucket size to stop this from happening), just remeasure the text dimensions on the fly (obviously much slower, but shouldn't theoretically occur more than a few times in a frame under the worst circumstances)