The Mac and iPhone maker will pour at least another $100 million into its new Austin campus, a filing shows. It’s welcome news for a city — and real estate sector — that has been put at risk by big tech employers lately as recession fears loom and the work-from-home model prevails. Facebook is slimming down in Austin, backing out of space it was to take in Austin’s next tallest skyscraper, while news of coming layoffs at Amazon.com Inc. and Google Inc. have many real estate pros on pins and needles.