More cosmetic improvements in the GLTF code

[aaronfranke]

• Fix an inconsistency of r_state vs p_state, inverts the change in Improve GLTF export logic for scene root nodes #79801, now it uses p_state.
• Improve documentation of GLTFDocument.
• Rename texture_dir to relative_texture_dir since it's relative to the GLTF file.
• Rename new_texture_dir to full_texture_dir since it's the full directory path.
• Rename Dictionary d to image_dict.
• Rename Dictionary d to texture_dict and Ref<GLTFTexture> t to gltf_texture.
• Remove unnecessary Area3D include.

Several of these were taken from #79314, split off for easier reviewing.

[akien-mga]

Thanks!