MVKCmdBeginRenderPass don't hold VkRenderPassBeginInfo.
Convert MVKCmdBeginRenderPass and MVKCmdExecuteCommands to template classes.
MVKCommandBuffer::recordBeginRenderPass() pass MVKLoadStoreOverrideMixin
instead of MVKCmdBeginRenderPass.
MVKCommandBuffer::recordEndRenderPass() pass nothing.
Add MVKAddCmdFromThreshold() macro in vulkan.mm.
Add MVK_CMD_TYPE_POOLS_FROM_THRESHOLD() macro in MVKCommandTypePools.def.