UI: Add simple output mode encoder fallback

Prevents simple output mode from getting stuck on an encoder that may
not be available suddenly for whatever reason (system device changes,
driver issues, etc).  If the encoder is no longer available, falls back
to x264 to ensure that the user can continue to use the program.

UI/window-basic-main.cpp

@@ -1479,6 +1479,8 @@ void OBSBasic::OBSInit()
 	blog(LOG_INFO, "---------------------------------");
 	obs_post_load_modules();
 
+	CheckForSimpleModeX264Fallback();
+
 	blog(LOG_INFO, STARTUP_SEPARATOR);
 
 	ResetOutputs();