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.
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