Remove concurrent access to objects from worker threads

[kkaefer]

Currently, we have a number of objects that are shared across threads. This means we're locking these objects when accessing them. To remove lock contention, and to align the architecture with JS, we should move away from shared memory objects.

The way JS (web workers) operates is by having a read-only state of the object in question (e.g. the sprite/glyph atlas). When a write access is required, the worker thread is suspended and the operation is carried out in the main thread (e.g. adding glyphs to the glyph atlas). Then, the updated state is sent to the worker thread again.

[ljbade]

Is there a way to do this nicely with C++, ie worker thread pausing and updating happens automatically when you call some write access method? (ie all all non const methods)

[ChrisLoer]

Fixed with #8341.