Convert API Commandline Switch

[andrew-lunarg]

Currently Convert operates equivalently to the replay flag --api=all but it would be more efficient for all current captures to use --api=vulkan or --api=dx12 since all of our captures currently encode a single API. Since there more APIs in PR (D3D11) it may make sense to have a flag per API with enable/disable to be able to support captures with multiple APIs interweaved (only if mixed-API captures is a reasonably likely future direction).

• --api-vulkan=[enable|disable] (default = enable)
• --api-dx12=[enable|disable] (default = disable)
• --api-dx11=[enable|disable] (default = disable)

In response to these flags Convert could connect up more or fewer Decoders and Consumers. If it connects fewer, it should run slightly faster. Maybe run some experiments as to the perf benefit before implementing a full solution with the flag wired up since Convert spends most of its time serializing JSON trees and destroying JSON trees.