VideoCommon: shader compiler workers below the emulation threads

The shader compiler's worker threads run at a lower priority than the
threads that emulate, on Windows (below normal) and Linux (nice 5): on
a machine with fewer cores than busy threads, what compiles in the
background waits, not the frame being emulated. Common gets
LowerCurrentThreadPriority for it.

Source/Core/Common/Thread.cpp

@@ -11,6 +11,11 @@
 #include <unistd.h>
 #endif
 
+#ifdef __linux__
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#endif
+
 #ifdef __APPLE__
 #include <mach/mach.h>
 #elif defined BSD4_4 || defined __FreeBSD__ || defined __OpenBSD__
@@ -43,6 +48,16 @@ void SetCurrentThreadName(const char* name)
   sthread_setname(name);
 }
 
+void LowerCurrentThreadPriority()
+{
+#ifdef _WIN32
+  SetThreadPriority(GetCurrentThread(), THREAD_PRIORITY_BELOW_NORMAL);
+#elif defined(__linux__)
+  /* Linux keeps a nice value per thread: this one only */
+  setpriority(PRIO_PROCESS, (id_t)syscall(SYS_gettid), 5);
+#endif
+}
+
 #ifndef _WIN32
 
 /* A thread's stack does not move: asked once per thread, as glibc's

Source/Core/Common/Thread.h

@@ -25,6 +25,11 @@ int CurrentThreadId();
 
 void SetCurrentThreadName(const char* name);
 
+// Below the threads that emulate, where the OS allows it: Windows and Linux.
+// For background work, which then takes no time from a frame while one is
+// being emulated, only what is left over.
+void LowerCurrentThreadPriority();
+
 // An sthread entry point that calls F on the object passed as the thread's userdata:
 //   m_thread = sthread_create(Common::ThreadEntry<&Foo::Run>, this);
 template <auto F>

Source/Core/VideoCommon/AsyncShaderCompiler.cpp

@@ -271,6 +271,9 @@ void AsyncShaderCompiler::WorkerThreadStart(void* worker)
 void AsyncShaderCompiler::WorkerThreadEntryPoint(Worker* worker)
 {
   Common::SetCurrentThreadName("AsyncShaderCompiler Worker");
+  /* what is compiled in the background waits; the frame being emulated
+   * does not, on a machine with fewer cores than busy threads */
+  Common::LowerCurrentThreadPriority();
 
   // Initialize worker thread with backend-specific method.
   if (!WorkerThreadInitWorkerThread(worker->param))