(WIP) More flexible image export handling for extensions.

[netpro2k]

There are now several GLTF extensions for alternate image formats like KHR_texture_basisu and EXT_texture_webp. It would be nice to add support for these to to other addons built against this extension without them having to duplicate the node traversal and channel combination/remapping logic already implemented in ExportImage.

This is a quick attempt at abstracting things just enough to allow that. I am not particularly happy with this solution but I think its a useful starting point for discussion.

Using this in the Hubs Blender Exporter to support .hdr (which blender already natively understands) files on lightmaps like this:
https://github.com/MozillaReality/hubs-blender-exporter/blob/76579e38ad2b3ad6768eef7b1acf575b2fe4302d/gather_properties.py#L181-L193
https://github.com/MozillaReality/hubs-blender-exporter/blob/76579e38ad2b3ad6768eef7b1acf575b2fe4302d/gather_properties.py#L215

For formats Blender does not already natively support you could instead completely override encode(mime):

ImageData.extension_for_mime["image/ktx2"] = ".ktx"
class BasisExportImage(ExportImage):
    def preferred_mime_type(self) -> str:
        return "image/ktx2"

    def encode(self, mime_type: Optional[str]) -> bytes:
        # use self.fills to generate a ktx2 image
        return bytes()

Again this feels a bit messy and its stretching the ExportImage abstraction quite a bit, but hopefully serves to show what an addon might want to be able to do. It would likely be a better solution to pull out the bits of gather_image and ExportImage that are generically useful and have the addon call those instead, skipping gather_image entirely, but I wanted to start with a more direct approach just to see the minimal set of changes needed.

---

Some related issues I ran into while looking into this: #1234 #1308

[netpro2k]

export_image_class appears to not be hashable which causes this to fail. Not ideal.

[netpro2k]

If the user specifically sets their export mode to jpeg, I think things should always export as a jpeg. When set to auto the new preffered_mime_type() check would naturally output as a png unless the input image was already a jpg

[netpro2k]

Attempting to remove as much as possible that is assuming a particular filetype. This check seems superfluous anyway.

[netpro2k]

We end up with multiple spots with very similar mappings... should be centralized somehow

[netpro2k]

The logic here is that all of the extensions related to image formats right now actually start at the texture level, so its likely they will be calling gather_image and can provide an override. Its possible this could instead be handled in reverse via something like a pre_gather_image_hook that could return some data, but it would likely require duplicating some functionality.

[netpro2k]

For comparison, this is the same thing but built against the existing exporter without modification. In many ways its cleaner, but there is some code duplicated (namely __encode_from_image) and I am intentionally skipping many edge cases (unsaved image, compositing multiple image channels, input images in a different format than export format) that ExportImage handles.

https://github.com/MozillaReality/hubs-blender-exporter/blob/hdr-lightmaps/gather_properties.py#L177