configuration: swap to per-game config memory for properties dialog

Does not set memory going in-game. Swaps to game values when opening the
properties dialog, then swaps back when closing it. Uses a `memcpy` to swap.
Also implements saving config files, limited to certain groups of configurations
so as to not risk setting unsafe configurations.

src/core/settings.cpp

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#include <cstring>
+
 #include "common/file_util.h"
 #include "core/core.h"
 #include "core/gdbstub/gdbstub.h"
@@ -65,6 +67,16 @@ Values global_values;
 Values game_values;
 Values *values = &global_values;
 
+void SwapValues(bool global) {
+    if (global) {
+        values = &global_values;
+    }
+    else {
+        std::memcpy(&game_values, &global_values, sizeof(global_values));
+        values = &game_values;
+    }
+}
+
 std::string GetTimeZoneString() {
     static constexpr std::array<const char*, 46> timezones{{
         "auto",      "default",   "CET", "CST6CDT", "Cuba",    "EET",    "Egypt",     "Eire",

src/core/settings.h

@@ -499,6 +499,7 @@ extern Values game_values;
 extern Values *values;
 
 float Volume();
+void SwapValues(bool global);
 
 bool IsGPULevelExtreme();
 bool IsGPULevelHigh();