Default texture should be transparent instead of white

[mzschwartz5]

This issue is related to several material image flashing issues: #1640 and #12532 (among others, probably).

A lot has been done to address these issues (see #12722 and #12767), but an issue remains that, on constructing a new Material, users may still see a white flash.

This has to do with the Update-loop-based image loading model that Material employs. Image loading is queued on construction but only kicked off on subsequent Update loop iterations. There's not a whole lot to be done there, if you can't use the async constructor (linked PR above).

One very easy mitigation would be to just make the default texture transparent instead of white. At least this way, materials won't flash white into existence before loading. The change would be super easy: simply change colors here to [0, 0, 0, 0]. (Admittedly, there might be a whole lot of unit tests to update after this... hopefully not, if we've structured things right).

Bonus: update the default cubemap in the same file.

[javagl]

On the one hand, I see the point: No ugly white flashes. On the other hand ... it could just turn that into an "ugly disappear-for-one-frame".

What may be more important: Iff the default texture is shown due to some ~"error condition", then - if I understood this correctly - this would lead to the object becoming invisible, which is nearly impossible to debug. (You don't know whether it's only due to the transparent texture, or because of some drawCommand/culling/... whatever issue).

(I actually changed the color to [255, 0, 255, 255] for some local tests. Magenta is not uncommon as an "error indicator"...)

[mzschwartz5]

I wouldn't be opposed to having a debug option for a different default texture color - best of both worlds?

[javagl]

I don't have a strong opinion, and it may boil down to whether the default texture is supposed to "emphasize" or "hide" that a certain texture/image was not found.

[Beilinson]

As a developer I would prefer the ability to choose (either globally or per image) whether the default texture is "hiding" or "emphasizing" the error as you said, maybe allow the error texture/color to be customizable I.e fallbackTexture/fallbackColor

[javagl]

The Fabric Schema currently states

Alternatively, a placeholder RGBA white texture can be used by specifying czm_defaultImage or czm_defaultCubeMap for textures or cube-maps, respectively.

^{(Emphasis by me)}

If the default is changed, this may have to be adjusted.