Libretro: Add frameskip option

CHANGES

@@ -54,6 +54,7 @@ Misc:
  - GB MBC: Remove erroneous bank 0 wrapping
  - GBA Cheats: Allow multiple ROM patches in the same slot
  - GB: Skip BIOS option now works
+ - Libretro: Add frameskip option
 
 0.6.1: (2017-10-01)
 Bugfixes:

src/platform/libretro/libretro.c

@@ -88,6 +88,13 @@ static void _reloadSettings(void) {
 		}
 	}
 
+	var.key = "mgba_frameskip";
+	var.value = 0;
+	if (environCallback(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value) {
+		opts.frameskip = strtol(var.value, NULL, 10);
+
+	}
+
 	mCoreConfigLoadDefaults(&core->config, &opts);
 	mCoreLoadConfig(core);
 }
@@ -105,6 +112,7 @@ void retro_set_environment(retro_environment_t env) {
 		{ "mgba_use_bios", "Use BIOS file if found (requires restart); ON|OFF" },
 		{ "mgba_skip_bios", "Skip BIOS intro (requires restart); OFF|ON" },
 		{ "mgba_idle_optimization", "Idle loop removal; Remove Known|Detect and Remove|Don't Remove" },
+		{ "mgba_frameskip", "Frameskip; 0|1|2|3|4|5|6|7|8|9|10" },
 		{ 0, 0 }
 	};
 
@@ -222,16 +230,22 @@ void retro_run(void) {
 	uint16_t keys;
 	inputPollCallback();
 
-	struct retro_variable var = {
-		.key = "mgba_allow_opposing_directions",
-		.value = 0
-	};
-
 	bool updated = false;
 	if (environCallback(RETRO_ENVIRONMENT_GET_VARIABLE_UPDATE, &updated) && updated) {
+		struct retro_variable var = {
+			.key = "mgba_allow_opposing_directions",
+			.value = 0
+		};
 		if (environCallback(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value) {
 			((struct GBA*) core->board)->allowOpposingDirections = strcmp(var.value, "yes") == 0;
 		}
+
+		var.key = "mgba_frameskip";
+		var.value = 0;
+		if (environCallback(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value) {
+			mCoreConfigSetUIntValue(&core->config, "frameskip", strtol(var.value, NULL, 10));
+			mCoreLoadConfig(core);
+		}
 	}
 
 	keys = 0;