OSX Catalina (10.15) Issues/Fixes

While OSX is not officially supported - I wanted to point out the issues and solutions I found necessary to get Deepmind lab running on 10.15 on the macos branch with python3 for those who are interested. I'm not sure if any of these should be merged, but the documentation could be adapted.

**Issues** (Or at least, those I was able to find)
1. **RESOLVED (AS OF 12/6)**: ~~The "//:deepmind/support:realpath" script no longer functions on 10.15. **Fix:** Install the coreutils brew package (which includes a functioning realpath) and replace all lines in BUILD of the form `ln -s $$($(location //deepmind/support:realpath) ...` with `ln -s $$(realpath ...`. The patch file is here: https://gist.github.com/DavidMChan/6654a82c658480f8b983b9afabe4502c~~

2. SDL 2.0.10 renders the window in only 1/4 of the screen. **Fix:** Use SDL 2.0.12 by installing with --HEAD on brew. This is related to high DPI support in the upstream (See: https://hg.libsdl.org/SDL/rev/46b094f7d20e and https://github.com/ioquake/ioq3/issues/422)