Fix GLTF texture filename decoding

[cesarizu]

This PR backports a specific fix to gltf filename decoding. GLTF files uri-encode references to external filenames. When importing into Godot, if the filename or a directory have any special characters, Godot fails to find the file. The solution is to decode the uri before looking for the file. On the master branch this is done using String::uri_decode but this is not available on 3.x. For these versions, and in this PR, I used String::percent_decode which covers most of the cases of uri encoding for GLFT files.

A more complete solution would be to backport the String::uri_(en|de)code methods and use those.

Test Project

gltf-tests.zip

Output without this PR:

Resource file not found: res://cube%2Btexture.png.
Can't open file from path 'res://cube%2Btexture.png'.
modules/gltf/gltf_document.cpp:3070 - glTF: Image index '0' couldn't be loaded as a buffer of MIME type 'image/png' from URI: res://cube%2Btexture.png. Skipping it.

[fire]

There's another percent_decode missing that exists in Godot master 4.

[cesarizu]

I think percent_(en|de)code and http_(en|de)code (also missing in 4.x), are partial methods of encoding/decoding strings for URIs and neither do what uri_decode does completely. I think it would be better to backport the 4.x method than to restore the 3.x methods.

[fire]

Oh I meant import has a flow and then export has one. I only saw one in this pr.

[akien-mga]

Oh I meant import has a flow and then export has one. I only saw one in this pr.

@fire was right, there was another similar case in _parse_buffer that had to be handled.

I force pushed a rebase to include it, fix another typo, and switch to http_unescape which is the closer equivalent to uri_decode in 3.x (see #43978).

[akien-mga]

Tested with the attached project, it works fine 👍

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.5.