Improve tracking of need for Metal visibility buffer in Metal renderpasses

[billhollings]

The current logic around deciding whether a visibility buffer is needed is relatively crude. Currently, it is applied to all render passes in a Vulkan command buffer based on at least one occlusion query being present. A Metal renderpass needs a visibility buffer if a query is started either before it or inside it. For a command buffer with multiple Metal render passes, not all of them may need it.

Recommended logic:

• Track need for visibility buffer in MVKCmdBeginRenderPass and MVKCmdNextSubpass.
• MVKCommandBuffer track when MVKCmdBeginRenderPass or MVKCmdNextSubpass added and set it's _needsVisibilityResultMTLBuffer from MVKCommandBuffer:: _isOcclusionQueryOpen.
• Adding, MVKCmdBeginQuery sets MVKCommandBuffer::_isOcclusionQueryOpen and if a MVKCmdBeginRenderPass or MVKCmdNextSubpass is open, set its _needsVisibilityResultMTLBuffer too.
• Adding MVKCmdEndQuery sets MVKCommandBuffer:: _isOcclusionQueryOpen to false.
• Adding MVKCmdEndRenderPass clears open render/sub pass in MVKCommandBuffer.

[billhollings]

I performed some further analysis and experimentation. Additional requirements:

• Vulkan secondary command buffers may contain queries that affect an open begin renderpass command in the primary buffer, and _needsVisibilityResultMTLBuffer is set in any of the secondary command buffers.
• MVKCommandEncoder:: _needsVisibilityResultMTLBuffer status set by encoding of MVKBeginRenderpassandMVKNextSubpass`.
• MVKCommandEncoder:: _needsVisibilityResultMTLBuffer status used in beginMetalRenderPass() to determine whether visibility buffer should be used.
• Metal renderpass iterations used for Vulkan multi view rendering should not change any flags, but should honor value of MVKCommandEncoder:: _needsVisibilityResultMTLBuffer.

[billhollings]

Since a full solution appears to be necessarily complex, and the benefits of the improvement are unclear (assumed to be some type of undefined performance improvement), at this point, further work on this is being left until a definite need arises.