fix base64 padding when loading a gltf file

[mockersf]

Objective

• After Update base64 requirement from 0.13.0 to 0.21.5 #10336, some gltf files fail to load (examples custom_gltf_vertex_attribute, gltf_skinned_mesh, ...)
• Fix them

Solution

• Allow padding in base 64 decoder

[mgi388]

Out of interest, more for my own curiosity, is this change a good candidate for a test to protect against further regressions?

[alice-i-cecile]

Yes, this is a good candidate for a regression test. It would involve working with assets though: I'm not sure of the best way to set that up.

[mockersf]

Now that asset v2 is done, finally working on testable asset loaders should happen

There is an extra safety net after Bevy CI: http://thebevyflock.github.io/bevy-example-runner/ which runs most examples regularly and check their screenshots. This is how I noticed the issue.