Add OpenXR loader init properties support

[hougantc-nvda]

• Support custom loader properties via lovr.conf initproperties table
• Fix null terminator buffer overflow in extension string allocation
• LOVR_BUILD_WITH_SYMBOLS now exports all symbols on Windows
• Add NVIDIA to contributors

[hougantc-nvda]

It seems you have other code for Android so I targeted xrInitializeLoaderKHR to desktop platforms.

[bjornbytes]

Given the xrInitializeLoaderKHR doesn't really depend on any of lovr's internal OpenXR stuff, is there a reason the loader properties need to be plumbed through lovr? Could you do something like require('myplugin').initLoader({ properties = {} }) in conf.lua and set up the loader before booting the rest of lovr? These initialization properties feel a bit too low-level / esoteric compared to the usual configuration lovr exposes.