UI: Check low disk space only if recording to a file

The Custom FFmpeg Output allows the user to configure it to output to a
URL instead of a file. In OBSBasic::StartRecording(), we unconditionally
call LowDiskSpace() to check for low disk space because we assume that a
"recording" will always be to disk. When os_get_free_disk_space() is
called on a non-existent path, it returns 0, which causes OBS to emit a
low disk space warning. Instead, only call DiskSpaceMessage() if not
"recording" to a URL.

UI/window-basic-main.hpp

@@ -881,6 +881,7 @@ private:
 	void UpdatePause(bool activate = true);
 	void UpdateReplayBuffer(bool activate = true);
 
+	bool IsFFmpegOutputToURL() const;
 	bool OutputPathValid();
 	void OutputPathInvalidMessage();