Fix glTF coordinate conversion not converting tangents

[greeble-dev]

Objective

Fix glTF coordinate conversion not converting tangents.

Report: https://discord.com/channels/691052431525675048/692572690833473578/1405362252617355335
Thread: https://discord.com/channels/691052431525675048/1405451520836898848

Solution

Fixed by removing a redundant copy of the tangent attributes.

In #5370 attribute copying was moved to a more generic function (see convert_attribute). But one code path was left behind, so the tangents are actually copied twice. This is technically a bug, but a harmless one since both copies are the same.

Later on, #19633 added the option to convert attribute coordinates. But only the first tangent copy (in convert_attribute) applies the conversion - the second copy will overwrite them with unconverted values. This PR removes the second copy.

There's also some minor code quality tweaks - prefer contains_attribute() to attribute().is_some(), and fixed some bad indentation in log macros. I can revert these if we want to play it safe.

Testing

Tested a modified version of example load_gltf with and without feature gltf_convert_coordinates_default. Also tested after hacking the loader to use the code path where tangents are recalculated.

[janhohenheim]

Adding to the milestone because this is needed for the glTF bonanza