fix layer offset calculation when uploading cubemaps with MipmapLayout

[KAlO2]

Pull Request Template

Description

Fixed per-layer buffer offsets when uploading images with MipmapLayout.

This fixes issue #1721

vsgExamples/vsgskybox.cpp renders skybox with incorrect faces.

So I used RenderDoc to capture frames and inspect the SkyBox textures. I found that when switching between the ±X/Y/Z faces, the displayed image content remained unchanged — in other words, all six faces were using the same texture. I traced the issue to an incorrect offset calculation in TransferTask.cpp.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test vsgExamples/vsgskybox.cpp

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• 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