Fix 2 ways libTAS incorrectly considers the movie modified and prompts for save

[Perdu]

• First commit introduces actually checking whether lua's onInput() callback modifies input instead of blindly guessing it always does
• Second commit properly reinitializes MovieFileInput, to avoid libTAS thinking the movie was modified from a previous execution (which happens if you launch movie, modify it, don't save, then open it again)