Fix visual bug with smooth font

[ShadowTzu]

Smooth font need to have a transparent material because the border has a pixel alpha channel under 1.0

[elisee]

Thanks for the PR! It looks like the patch can be simplified to:

    mesh.material.transparent = this.font.filtering === "smooth";

But I'm wondering if this is the correct fix. Shouldn't we just add an Opacity field like in the sprite and model editors / renderer components editors? cc @bilou84

[elisee]

Looks like this is indeed what we had planned: https://trello.com/c/RpdoFljI/683-font-add-opacity-to-component-editor, so let's just do that soon instead.

[ShadowTzu]

With a smooth filter there is no case with disabled opacity that is useful