more multi-thread fixes

don't use static on acquire()
don't yield at all
run fillBuf() on thread
fix option tooltip as it introduces some overhead

src/engine/platform/amiga.cpp

@@ -80,7 +80,7 @@ const char** DivPlatformAmiga::getRegisterSheet() {
   }
 
 void DivPlatformAmiga::acquire(short** buf, size_t len) {
-  static int outL, outR, output;
+  thread_local int outL, outR, output;
   for (size_t h=0; h<len; h++) {
     bool hsync=bypassLimits;
     outL=0;