Patch Notes: Rarer gems

Loot

The graded gems you set into sockets (Chipped, Flawed, Flawless and Perfect stones, plus plain ones found the same way) now drop about a third as often from bodies, dungeon piles and tavern or guild markers. A mid-level foe drops one about once every 33 kills instead of once every 12. In the deepest dungeons and from boss-level foes, a gem drops once every 20 sources instead of once every 7.
A gem's grade still depends on where you find it, so a deep source still gives the Flawless and Perfect stones.
No change to the gems world bosses drop, the gems shops sell, Daggerfall's own gem drops, or the gems you mine.

Lamp shadows in towns at night take less work. When nothing near a lamp has moved, it reuses last frame's answer about which nearby plants and trees can cast into it. The shadows are exactly the same.
Distant small flats are skipped before the game checks whether they are on screen, not after.
The glow pass skips flats that have no glow before checking whether they are on screen.
Towns use less graphics memory. The large shared buffers for trees, plants and lamps now share one index buffer, where each had its own copy. That saves about 6 MB per buffer, and opening a new one no longer causes a short hitch.

PERF-VOL + PERF-ORDER, and AUDIT PERF-ARENA: lamp-shadow answers kept, cheapest skip first, one index buffer for every arena by @Lattymoy in #770
GEM-RARE: the world's gem find a third of GEM2's by @Lattymoy in #772

Full Changelog: app-v0.1.7791...app-v0.1.7804