Handle URI with spaces in I3DM

[javagl]

Addresses #276 :

Until now, the validator handled the case that the URI inside an I3DM was padded with 0x00 (zero) characters. But the specification says

Otherwise, if the glTF field is a UTF-8 string, it shall be padded with trailing Space characters (0x20) to satisfy alignment requirements of the tile, which shall be removed at runtime before requesting the glTF asset.

So the validator has to handle the case that the URI is padded with 0x20 (space) characters.

One question is still open: Should the validator consider it as an ERROR when the URI (i.e. the payload) is padded with 0x00 characters instead of spaces?

Right now, it silently allows that. But sticking to the specification text, I'd say "Yes, this should be an error!".

If this is confirmed, I'd update this PR to treat this as an error (and this aspect has to be reviewed in the 3d-tiles-tools in the context of CesiumGS/3d-tiles-tools#45 )

[javagl]

(Note: Depending on the last decision here, the change log still has to be updated)

[javagl]

Treating trailing 0x00 bytes as an ERROR would potentially cause existing (formerly "valid") files to become invalid. This may be done in the future, but only after carefully evaluating the implications.

For now, the change from this PR is:

• Any 0x00 (zero) bytes in the URI buffer cause a WARNING
• Trailing 0x20 (space) bytes are properly trimmed

Fixes #276