This eliminates the reliance on ThreadContext to provide GLObjectStore, and statically enforces that GL cleanup functions happen only when GLObjectStore::performCleanup is called.
With the elimination of the Map thread, this becomes important because there may be multiple GLObjectStore's per-thread, and Map will need to ensure that the correct context is active when calling GLObjectStore::performCleanup.
Texture pool now allocates a block of 64 textures per time. Each block
contains a TextureObject which handles GL object lifetime. Blocks can be
added or removed according to the following rules:
1. If pool is empty, created a new block upon first call (lazy load);
2. Iterate blocks to find an available texture ID:
- If found, remove that from the list of available IDs for that block
and return its ID.
- Othewise, allocate a new block and grab an available ID from that.
3. When releasing a texture ID, check if the associated block is free.
If no IDs are currently in use, then abandon its TextureObject and
remove the block from the pool.
When inserting an cached resource, or removing a region, remove least-recently used resources and tiles, not used by offline regions, until the used database size, as calculated by multiplying the number of in-use pages by the page size, is less than the maximum cache size minus 5 times the page size.
In addition, OfflineDatabase may be configured to ignore cache puts of individual resources larger than a certain size.
This policy is similar but not identical to the former SQLiteCache policy:
* It accounts for offline, by exempting resources required by offline regions from eviction.
* It must delete from two tables (resources and tiles), rather than one. Currently the strategy is naive: evict 50 rows at a time from each table.
* It makes maximumCacheSize and maximumCacheEntrySize completely independent. The SQLiteCache implementation evicted when `usedSize > maximumCacheSize - 2 * maximumCacheEntrySize`. This evicts when `usedSize > maximumCacheSize - 5 * pageSize`.
* It uses a non-unlimited default value for maximumCacheSize: 50 MB. We should have always had a limit in place; "a cache without an eviction policy is a resource leak".
* Use "named constructors": empty, world, hull
* Make the two-argument constructor lenient (i.e., it is a hull operation)
* Add various accessors
* Enforce a single empty representation
