macOS support

[didzis]

Adds macOS support remaining compatible with other platforms, i.e., intended to be merged with master branch. Works for all three modes: non-headless, headless with hardware acceleration and headless with software rendering. Build instructions found in docs/build_macos.md. This addresses #7, #39 and possibly #22.

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CLAs look good, thanks!

[tkoeppe]

sendfile is available on BSD/MacOSX as well, e.g. https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/sendfile.2.html. It might have a slightly different signature than on Linux, but we shouldn't need to reimplement it.

[tkoeppe]

Hi! Are you still interested in this? I've put together a few smaller changes in the macos branch of this repository. At the top of that branch there's a commit that adds your new headless renderer, but that doesn't quite seem to work yet. If you like, feel free to send patches against that branch (e.g. by rebasing this pull request branch). I think all we need to do is fix up headless_macos_glimp.c.