Fixed Z3 RAM would not get set correctly on reset

- Fixes bug where workbench background would be corrupt when switching from one RTG config to another
- Fixes CDTV would not show kickstart screen #658

src/osdep/amiberry.cpp

@@ -1301,15 +1301,32 @@ void target_addtorecent(const TCHAR* name, int t)
 {
 }
 
-
-void target_reset(void)
+void target_reset()
 {
 	clipboard_reset();
 
 	auto* const ad = &adisplays;
 	ad->picasso_requested_on = false;
 	ad->picasso_on = false;
-
+
+	if (currprefs.z3fastmem[0].size != changed_prefs.z3fastmem[0].size)
+	{
+		currprefs.z3fastmem[0].size = changed_prefs.z3fastmem[0].size;
+		z3fastmem_bank[0].reserved_size = 0;
+		z3fastmem_bank[0].mask = 0;
+		z3fastmem_bank[0].baseaddr = NULL;
+
+		mapped_free(&z3fastmem_bank[0]);
+
+		z3fastmem_bank[0].allocated_size = 0;
+		z3fastmem_bank[0].reserved_size = currprefs.z3fastmem[0].size;
+		if (currprefs.z3fastmem[0].size) return;
+
+		z3fastmem_bank[0].mask = currprefs.z3fastmem[0].size - 1;
+		z3fastmem_bank[0].label = _T("*");
+		mapped_malloc(&z3fastmem_bank[0]);
+		changed_prefs.z3fastmem[0].size = currprefs.z3fastmem[0].size;
+	}
 }
 
 bool target_can_autoswitchdevice(void)