Minor Release - Upgrade to OptiX 6.0.0 and add maxmem option

It is finally time to get this release out now that the new OptiX is out! This release includes,

• Upgraded to OptiX 6.0.0.
  • Less DLLs to ship
  • Turing architecture (RTX) support.
  • See https://developer.nvidia.com/optix for full list of changes.
• Added a -maxmem command line flag for setting a maximum memory limit in MB for the denoiser to use.
  • From the OptiX programming guide - "The denoiser will try to stay below the defined size by splitting the image to be denoised into tiles and denoising them individually. If the defined memory size does not allow denoising, the denoising operation will fail and leave the image in its original state. However, setting a maximum memory size may result in slower denoising performance.".
• Fixed a bug where feature AOVs would be read incorrectly if they were a different format to the beauty AOV #10 - thanks @jackyjacky1307 for finding this bug and the solution to this.

IMPORTANT - As with the upgrade to OptiX 6.0.0 you will now require an Nvidia driver of at least 418.xx or higher and an Nvidia GPU of Maxwell architecture (Geforce 700 series) or higher.