[Backport] Enable alpha based transparency on PBR materials by default #249

[chapulina]

Backporting #249 to improve transparent materials on Citadel.

It changes the default behavior, but most likely no users are using materials with alpha yet. That's the hope 🤞

[luca-della-vedova]

Ah in hindsight this needs the SetAlphaFromTexture function that was added in ign-rendering4 for Dome so it wouldn't compile without backporting that PR as well.
Since the BaseMaterial class doesn't use PIMPL I believe the addition of the needed member variable that was needed would break ABI, so it is probably not possible to backport this PR?

[chapulina]

Ahh you're right 😢 That's probably why it initially targeted Dome... 😕