Handle multiple draws per frame better

[alexheretic]

Currently a GlyphBrush should call process_queued once per frame for the caching system to work properly. Multiple draws can be handled with multiple GlyphBrush each drawing once per frame.

We could probably do better here though.

[Nuckal777]

Didn't look deeply into the topic. Can we achieve better cache usage with multiple draws, if the user calls GlyphBrush::cleanup_frame() explicitly when finishing a frame? Currently the cache is trimmed after each draw implicitly.

[alexheretic]

Moved to more concrete issues #157 & #158