GPU CI and visual regression tests

[asinghvi17]

You can now load a subset of MakieGallery's database, so it's easy to set up tests for specific packages. I'm opening this issue to remind me to actually do it.

Tasks to do for this:

• Add the Gitlab template file
• Create a Docker image based off of OpenGL + Makie essential libraries + artifacts
• Modify the test script to run MakieGallery tests as well.
• Optional: add more StatsMakie examples

cc @piever