rsx: Misc optimizations and fixes

[kd-11]

• Avoid unordered_map insert/erase in hot loop. Use a ring allocation instead. This was an issue in ZCULL code where frequent insert/erase cycles were wasting performance.
• Avoid malloc/realloc/free in the vertex analysis routine. This method is called several times for every draw call and the cost was adding up quite a bit.
• Elide contention by aborting FIFO puller cache load if we have at least 50% of the data already loaded. Since the usecase for this is a "chase" situation, it is a far better use of resources to let CELL run ahead further and reduces time wasted spinning. This speeds up the FIFO read routine by ~12% when using atomic FIFO.
• Fix image concurrency access when using async compute with vulkan.

Overall, this set of changes gives a ~10% uplift on average which is nice.