Drops the least recently used entries of the render caches until they hold at most maxBytes together:
decoded content first (only builds read it), then images, glyph outlines and, last, display lists. Nothing is disposed; a
dropped entry is rebuilt at its next use and renders the same.
public long TrimCaches(long maxBytes)
The most bytes the caches may keep; zero empties them.
Spec #33 viewer capability 13: one memory budget covers a viewer's tiles, its display lists and the core caches, so the viewer trims the caches to what its tiles leave. Safe to call while other threads render.