Make sure getTexture works in vertex shaders

[davepagurek]

Resolves #8416

Changes:

• Adds the getTexture definition to GLSL shaders regardless of whether they're vertex or fragment
• This forces all internal GLSL shaders to use the webgl2CompatibilityPrefix function. The image light shaders didn't do this yet, so this updates them to use that system.
• Adds a unit test showing reading data out of a texture in a vertex shader hook

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated