--Address HBAO image transparency oddness; remove unused variable.

[jturner65]

Motivation and Context

Image captures of scenes containing HBAO effect were demonstrating strange transparency-related artifacts due to the HBAO effect also being present in the alpha channel and an improper blend being performed. This PR addresses this by correcting the blend mechanism. New Test/ground truth images are also generated. Lastly, an unused value is removed.

How Has This Been Tested

C++ and python tests pass locally. New test images are synthesized to account for the new blend algorithm for the HBAO output framebuffer.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[aclegg3]

Thanks @jturner65 and @mosra for fixing this so fast! 💪