Fix: Enhance OBJ parser to support variable face formats (v//vn, v/vt)

[TeAmo-Lemon]

Description:

I encountered an issue where the model loader failed to parse OBJ files that use the v//vn format (vertex and normal, but no texture coordinates) or other variations. The original code strictly expected a v/vt/vn structure, causing read errors on models with different export settings.

What this PR does:
The current OBJ face parser assumes every face vertex always has vertex/texcoord/normal indices in the format f v/t/n v/t/n v/t/n.
It crashes or fails silently on OBJ files that use the common optional format, such as:

f 815//815 869//869 868//868

(only vertex and normal, texture coordinate is missing → double slash //)

Changes

• Modified the face parsing loop to properly handle cases where texture coordinate or normal is omitted
• Implemented sscanf checks to dynamically detect the format of each face vertex (v/vt/vn, v//vn, v/vt, or just v).
• Assigned default indices (1) to prevent undefined behavior before decrementing.
• Indices are still correctly decremented (--f, --t, --n) only when present.

Before:

• Cannot parse f v//vn lines → parsing error or wrong indices

After:

Correctly parses lines like:

• f 1/1/1 2/2/2 3/3/3 (full)
• f 1//1 2//2 3//3 (no texcoord)
• f 1/1 2/2 3/3 (no normal)
• f 1 2 3 (only position)

Tested with:

• Original tiny renderer models
• Several community OBJ files containing // syntax

Thanks for reviewing!