Reimplement Backend_Delay to fix threads freezing

So our workaround for making usleep thread-safe mostly worked in that
it fixed one of the things making it thread-unsafe, but it seems that
sigsuspend, which usleep uses, is also not thread-safe, since
sometimes one of our two threads would freeze, stuck inside
sigsuspend. So instead of usleep, we're using a trick that Stacken
member map suggested: using select. Calling select with no file
descriptors to work on will have it block on the kernel level until
its timeout is hit. This acts as a sleep. As it turns out, this seems
to completely solve our sleep-related problems, and massively reduces
the amount of audio buffer underruns as a nice bonus!

src/Backends/Platform/X11.cpp

@@ -158,5 +158,9 @@ unsigned long Backend_GetTicks(void) {
 }
 
 void Backend_Delay(unsigned int ticks) {
-    usleep(ticks * 1000);
+    struct timeval timeout;
+    timeout.tv_sec = ticks / 1000;
+    timeout.tv_usec = (ticks * 1000) % 1000000;
+
+    select(0, NULL, NULL, NULL, &timeout);
 }