Move renderSamples() to DivDispatch implementations.

To prevent rendering samples for systems that are not in use.

Additionally, it gives the systems more flexibility to render the samples
according to their specific configuration.

src/engine/platform/abstract.cpp

@@ -141,6 +141,22 @@ const char** DivDispatch::getRegisterSheet() {
   return NULL;
 }
 
+const void* DivDispatch::getSampleMem(int index) {
+  return NULL;
+}
+
+size_t DivDispatch::getSampleMemCapacity(int index) {
+  return 0;
+}
+
+size_t DivDispatch::getSampleMemUsage(int index) {
+  return 0;
+}
+
+void DivDispatch::renderSamples() {
+  
+}
+
 int DivDispatch::init(DivEngine* p, int channels, int sugRate, unsigned int flags) {
   return 0;
 }