Per-eye distortion correction

[russell-taylor]

Added the ability to override the per-HMD distortion parameters with per-eye distortion parameters to support a vendor that has custom calibration per eye on their display.

Decided to add a per-eye optional distortion override based on the following analysis:

• Make the hmd distortion entry in always an array: - Would break all existing configuration files.
• Make the hmd distortion entry into an optional array: + Only one place in the configuration file to specify distortion; + Backwards compatible with earlier config files; - May be mismatch between # eyes and # distortion (maybe more distortion than eyes...).
• Add an optional override distortion entry in each eye description: + Would have as many places to specify as there are eyes, so no mismatch; + Backwards compatible with earlier config files; - Would provide two different places to specify the distortion (hmd and eye).

Verified that this was backwards compatible with existing config and that it works with different distortion per eye on a new config.

[russell-taylor]

Thanks for the review and merge! Indeed, of the changes in the request were verified on Windows, Linux, and Mac.