Add thread priority API

Expose an API to change the priority of the current thread.

app/src/compat.h

@@ -54,6 +54,10 @@
 # define SCRCPY_SDL_HAS_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR
 #endif
 
+#if SDL_VERSION_ATLEAST(2, 0, 16)
+# define SCRCPY_SDL_HAS_THREAD_PRIORITY_TIME_CRITICAL
+#endif
+
 #ifndef HAVE_STRDUP
 char *strdup(const char *s);
 #endif