set thread priority on Linux

platform/default/thread.cpp

@@ -1,11 +1,23 @@
 #include <mbgl/platform/platform.hpp>
 
+#include <mbgl/platform/log.hpp>
+
+#include <pthread.h>
+#include <sched.h>
+
 namespace mbgl {
 namespace platform {
 
 void makeThreadLowPriority() {
-    // no-op
+#ifdef SCHED_IDLE
+    struct sched_param param;
+    param.sched_priority = 0;
+    int status = sched_setscheduler(0, SCHED_IDLE, &param);
+    if (status != 0) {
+        Log::Warning(Event::General, "Couldn't set thread scheduling policy");
+    }
+#endif
 }
 
-}
-}
+} // namespace platform
+} // namespace mbgl