libretro: Offer "Up to 2" buffered frames of graphics commands again

6bb91bf2f0 removed the option along with lowering the default. The default stays at "Up to 1"; this only
brings back the choice, which the standalone app still has.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libretro/libretro.cpp

@@ -816,6 +816,8 @@ static void check_variables(CoreParameter &coreParam)
          g_Config.iInflightFrames = 1;
       else if (!strcmp(var.value, "Up to 1"))
          g_Config.iInflightFrames = 2;
+      else if (!strcmp(var.value, "Up to 2"))
+         g_Config.iInflightFrames = 3;
    }
 
    var.key = "ppsspp_depth_raster";

libretro/libretro_core_options.h

@@ -436,6 +436,7 @@ struct retro_core_option_v2_definition option_defs_us[] = {
       {
          { "No buffer", NULL },
          { "Up to 1",   NULL },
+         { "Up to 2",   NULL },
          { NULL, NULL },
       },
       "Up to 1"