Add idle thread priority option, use it in shader compilation threads.

src/common/rt64_thread.cpp

@@ -18,6 +18,8 @@ namespace RT64 {
 #   if defined(_WIN32)
     static int toWindowsPriority(Thread::Priority priority) {
         switch (priority) {
+        case Thread::Priority::Idle:
+            return THREAD_PRIORITY_IDLE;
         case Thread::Priority::Lowest:
             return THREAD_PRIORITY_LOWEST;
         case Thread::Priority::Low:

src/common/rt64_thread.h

@@ -10,6 +10,7 @@
 namespace RT64 {
     struct Thread {
         enum class Priority {
+            Idle,
             Lowest,
             Low,
             Normal,

src/render/rt64_raster_shader_cache.cpp

@@ -141,8 +141,8 @@ namespace RT64 {
     void RasterShaderCache::CompilationThread::loop() {
         Thread::setCurrentThreadName("RT64 Shader");
 
-        // The shader compilation thread should have the lowest priority by default as the application can use the ubershader in the meantime.
+        // The shader compilation thread should have idle priority by default as the application can use the ubershader in the meantime.
-        Thread::setCurrentThreadPriority(Thread::Priority::Lowest);
+        Thread::setCurrentThreadPriority(Thread::Priority::Idle);
 
         threadRunning = true;