prepare acquireDirect API

provide direct access to blip_buf to dispatch code, which may allow for some serious optimizations

src/engine/platform/abstract.cpp

@@ -23,6 +23,9 @@
 void DivDispatch::acquire(short** buf, size_t len) {
 }
 
+void DivDispatch::acquireDirect(blip_buffer_t** bb, size_t off, size_t len) {
+}
+
 void DivDispatch::fillStream(std::vector<DivDelayedWrite>& stream, int sRate, size_t len) {
 }
 
@@ -127,6 +130,10 @@ bool DivDispatch::getDCOffRequired() {
   return false;
 }
 
+bool DivDispatch::hasAcquireDirect() {
+  return false;
+}
+
 bool DivDispatch::getWantPreNote() {
   return false;
 }