Disconnect other cameras when switching to PixelCamera

[ffreyer]

If you do something like this

fig = Figure()
scene = LScene(fig[1, 1], show_axis = false)
campixel!(scene.scene)
scatter!(scene, 400rand(10), 400rand(10))
fig

and then click (and drag) the plot area everything will disappear. That happens because campixel! does not disconnect the previous camera, in this case Camera3D. The pr fixes that.

[ffreyer]

Failing tests are from missing/old refimages. We can merge this