glTF: Fix parsing image data with mimeType undefined

[akien-mga]

The glTF 2.0 spec only makes mimeType mandatory for bufferView image data,
so the previous logic to handle URIs with base64-encoded images could fail if
mimeType is undefined.

The logic was documented and refactored to better handle the spec, notably:

• uri and bufferView are now mutually exclusive, and only the latter fails
  if mimeType is undefined.
• uri with a file path will now respect the mimeType if defined, and thus
  attempt loading the file with the specified format (even if its extension is
  not the one expected for this format). So we can support bad extensions (PNG
  data with .jpg extension) or custom ones (PNG data in .img file for
  example).
• uri with base64 encoded data will infer MIME type from data:image/png or
  data:image/jpeg if it was not documented in mimeType initially.
• uri with base64 encoded data, no mimeType and application/octet-stream
  or application/gltf-buffer will fall back to trying both PNG and JPEG
  loaders.

Fully fixes #33796 (and fixes up #42501).
Fixes #37514.

[fire]

fixes: #37514

[akien-mga]

I did some additional testing and it seems to work well. I can import the whole https://github.com/KhronosGroup/glTF-Sample-Models repo with one exception:

The BrainStem model triggers this crash #42569, so it should be removed or ignored before importing the repo. Same thing with its DAE version in sourceModels.

[akien-mga]

Tested a local cherry-pick on 3.2 and it also seems to import the whole repo of Khronos samples fine (including BrainStem).

[akien-mga]

Cherry-picked for 3.2.4.