OAM Order Hijack Support (to fix #227 and similar)

libretro/libretro.c

@@ -91,6 +91,7 @@ struct retro_perf_callback perf_cb;
 int dynarec_enable;
 boot_mode selected_boot_mode = boot_game;
 int sprite_limit = 1;
+int oam_hijacking_enabled = 0;  // Default to disabled
 
 static int rtc_mode = FEAT_AUTODETECT;
 static int rumble_mode = FEAT_AUTODETECT;
@@ -918,6 +919,17 @@ static void check_variables(bool started_from_load)
          sprite_limit = 0;
    }
 
+   var.key                = "gpsp_oam_hijack";
+   var.value              = 0;
+
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+   {
+      if (strcmp(var.value, "disabled") == 0)
+         oam_hijacking_enabled = 0;
+      else if (strcmp(var.value, "enabled") == 0)
+         oam_hijacking_enabled = 1;
+   }
+
    var.key                = "gpsp_frameskip";
    var.value              = 0;
    frameskip_type_prev    = current_frameskip_type;

libretro/libretro_core_options.h

@@ -100,6 +100,17 @@ struct retro_core_option_definition option_defs_us[] = {
       },
       "disabled"
    },
+   {
+      "gpsp_oam_hijack",
+      "OAM Priority Hijacking",
+      "Support for OAM priority hijacking behavior where objects relative BG priority does not follow the normal OAM order priority. The GBA has a transparency bug in these situations which is used by some games (Golden Sun, Golden Sun 2, Zelda: Minish Cap, Mario Golf etc), this option emulates that bug.",
+      {
+         { "disabled", NULL },
+         { "enabled",  NULL },
+         { NULL, NULL },
+      },
+      "disabled"
+   },
    {
       "gpsp_rtc",
       "RTC support",