FIX: handle change aspect change when render area is not full window.

[theodoregoetz]

Description

When the camera viewport is not the full window, the aspect ratio of the perspective projection matrix needs to be adjusted in the same way as the EllipsoidPerspective case.

Type of change

This is a behavior-changing fix since the aspect ratio was incorrectly calculated when the viewport extent is not exactly the same as the window extent.

How Has This Been Tested?

I have tested within my own framework built on top of VSG. I could include a minimal example, but I feel it is unneccessary because VSG already has the correct code in EllipsoidPerspective. This PR aligns the behavior of Perspective::changeExtent() and EllipsoidPerspective::changeExtent().

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

I have considered the following items, but believe no action needs to be taken

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works