proposal: selected geometry is highlighted with xray outline.

[mjkkirschner]

Purpose

This PR uses a standard helix post effect shader to display hidden geometry (occluded) with an xray outline post effect. Note that in newer versions of helix the color is more saturated - in this pr I've scaled down our selection color by .5f because the shader multiplies the color and makes it quite bright. We'll have to evaluate the color changes when moving to new helix versions.

The highlight works for points, meshes and lines.

PR also removes some dead references to dlls we no longer require.

TODO -

• [ ] new tests
• [ ] update image comparison tests that will now fail.

Declarations

Check these if you believe they are true

• [ ] The codebase is in a better state after this PR
• [ ] Is documented according to the standards
• [ ] The level of testing this PR includes is appropriate
• [ ] User facing strings, if any, are extracted into *.resx files
• [ ] All tests pass using the self-service CI.
• [x] Snapshot of UI changes, if any.
• [x] Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• [ ] This PR modifies some build requirements and the readme is updated

[aparajit-pratap]

@mjkkirschner what was it you were saying about the x-ray outline not appearing on a white background preview using the current helix version?

[mjkkirschner]

The way the color is blended with a white background has been changed in later versions of helix so that the blended color is less bright. Today it becomes white and is not very visible on a bright background.