Use multiple threads to load textures asynchronously.

[j-jorge]

This PR modifies TextureCache to use multiple threads to handle the work of AddImageAsync. The number of threads is set by std::thread::hardware_concurrency().

TextureCache now uses std::unique_ptr to make clear the ownership of data structures related to asynchronous texture loading. Redundant field _asyncRefCount has been removed. Uses of std::deque has been replaced by the more straightforward std::vector.

I've tested this in my app on my phone as well as, with ThreadSanitizer enabled, on my laptop.