Convert R8G8B8 textures to R8G8B8A8 when importing for better compatibility with GPUs

[expenses]

If you search for R8G8B8_ on either https://vulkan.gpuinfo.org/listlineartilingformats.php or https://vulkan.gpuinfo.org/listoptimaltilingformats.php you'll see that few GPUs are capable of using them in any meaningful way:

No Nvidia or AMD GPUs can sample them, for example: https://vulkan.gpuinfo.org/listdevicescoverage.php?lineartilingformat=R8G8B8_SRGB&featureflagbit=SAMPLED_IMAGE&platform=all.

This is a problem when importing glTF files, as the referenced images often do not have alpha channels. The solution I've come up with here is to remove the R8G8B8 format and convert DynamicImage::ImageRgb8s to RGBA instead.

[FrankenApps]

I don't see a reason why the library itself should do this. The user can always convert his image to rgba and then use DynamicImage::ImageRgba8(_) if he wants the better GPU support. This library should instead adhere to the gltf spec.

[expenses]

Well the main reason for having this in the crate is that someone would need to convert the loaded gltf::image::Data structs into image::RgbImages and then convert them to RGBA. It'd be a lot nicer to convert them when they're image::DynamicImages in the crate. Even if this isn't the default functionality, it would be nice to have it as a helper function.

[FrankenApps]

I see. I guess a helper function would make sense then, but I personally would not like the library to do this by default.

[aloucks]

The library should not do this without the user explicitly requesting the image conversion. A helper function would be fine but conversion should not happen by default.

[expenses]

I can't find a good way to do this nicely in the crate, but it's not too bad to do it the way I mention above, so I'm going to close this for now.