Check clearing of 3d textures when rendering to a slice (#4625)

* Check clearing of 3d textures when rendering to a slice

There was an issue in some browsers where, when rendering
to a slice of a 3d texture, they'd mark the entire texture
as initialized even though only the individual slice was
initialized.

Updated the tests so they check the entire texture.
Also updated them to try every format.

Author: greggman