Vulkan: Make multithreaded rendering an option.

Multi-threaded rendering is good for smoothing out performance spikes
and increasing performance on low-power devices. However, the way we use
it it doesn't benefit latency (input lag) at all, rather the opposite.

So make it an option.

Core/Config.cpp

@@ -611,6 +611,8 @@ static const ConfigSetting graphicsSettings[] = {
 	ConfigSetting("InflightFrames", &g_Config.iInflightFrames, 3, CfgFlag::DEFAULT),
 	ConfigSetting("RenderDuplicateFrames", &g_Config.bRenderDuplicateFrames, false, CfgFlag::PER_GAME),
 
+	ConfigSetting("MultiThreading", &g_Config.bRenderMultiThreading, true, CfgFlag::DEFAULT),
+
 	ConfigSetting("ShaderCache", &g_Config.bShaderCache, true, CfgFlag::DONT_SAVE),  // Doesn't save. Ini-only.
 	ConfigSetting("GpuLogProfiler", &g_Config.bGpuLogProfiler, false, CfgFlag::DEFAULT),
 };