Support object-form texture references in block models (1.21.4+)

[NereusOcean]

Since 1.21.4, a block model's textures value can be an object
{ "sprite": "...", "force_translucent": true } instead of a bare path string
(e.g. glass, stained glass). BlockModel.getTexture assumed a string and failed
to resolve these, so those blocks rendered untextured.

This resolves the object form to its sprite. Backward compatible - the string
form is unchanged. Translucency is intentionally out of scope: deepslate derives
it from block flags (getBlockFlags().semi_transparent), not from the model, so
force_translucent is not wired into rendering here.

Adds test/render/BlockModel.test.ts (string ref, object ref, ref-chain in object).

[misode]

Can the sprite value not have a # texture reference?

[NereusOcean]

Yeah, it can - the format allows # refs anywhere. In the current vanilla assets the sprite is always a plain path so it doesn't actually come up, but I updated getTexture so it keeps resolving when the sprite is a # reference (and when an object shows up partway through a chain).

I also swapped the while for a loop bounded by the number of texture keys - a valid chain can't hit the same key twice, so this way a cyclic reference in a broken pack can't spin forever. Added tests for both.

This might be overkill though - cyclic refs only happen in a broken pack. If you'd rather keep it minimal I can drop that last commit and leave just the object-sprite fix. Your call.

[misode]

I've fixed the issue in aa64176