Added support for vertex colors

[HeDo88TH]

Add support for OBJ files with per-vertex colors (extended vertex format: v x y z r g b).

Vertex colors are:

• Parsed from OBJ files by all three internal parsers (splitting, decimation, and glTF conversion stages)
• Preserved through mesh splitting (with correct interpolation at edge intersections) and LOD decimation
• Exported to glTF/GLB as COLOR_0 attribute with automatic sRGB → linear RGB conversion per the glTF specification

Closes #43

[Copilot]

Pull request overview

Adds end-to-end support for per-vertex colors in extended OBJ vertex lines (v x y z r g b) across the Obj2Tiles pipeline (splitting, decimation, and OBJ→glTF conversion), preserving/interpolating colors during splitting and exporting them as COLOR_0 in glTF (with sRGB→linear conversion).

Changes:

• Parse and carry optional vertex colors through Obj2Tiles.Library meshes (Mesh/MeshT) including split-time interpolation and OBJ write-back.
• Extend decimation-stage OBJ parsing/writing (ObjMesh) and pass colors through MeshDecimatorCore.
• Extend Obj2Gltf OBJ parser + glTF writer to emit COLOR_0 and write a dedicated colors buffer; add NUnit coverage for library-layer behavior.

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents vertex color support and cleans up formatting.
Obj2Tiles/Stages/Model/ObjMesh.cs	Adds vertex color parsing/storage + OBJ writing for the decimation-stage parser.
Obj2Tiles/Stages/DecimationStage.cs	Passes colors into/out of MeshDecimatorCore during LOD decimation.
Obj2Tiles.Library/Materials/RGB.cs	Adds RGB interpolation + sRGB→linear helper + equality members.
Obj2Tiles.Library/Geometry/MeshUtils.cs	Parses per-vertex RGB from OBJ and plumbs into Mesh/MeshT.
Obj2Tiles.Library/Geometry/Mesh.cs	Stores optional vertex colors; preserves + interpolates through splitting; writes colors in OBJ output.
Obj2Tiles.Library/Geometry/MeshT.cs	Same as Mesh but for textured meshes; updates split + OBJ writing + unused-vertex removal.
Obj2Tiles.Library.Test/VertexColorTests.cs	Adds tests for parsing/splitting/interpolation/round-trip OBJ writing with colors.
Obj2Tiles.Library.Test/TestData/triangle-colors.obj	New colored OBJ fixture.
Obj2Tiles.Library.Test/TestData/cube-colors.obj	New colored OBJ fixture.
Obj2Tiles.Library.Test/TestData/cube-colors-textured/*	New textured + colored OBJ/MTL (and texture) fixtures.
Obj2Tiles.Library.Test/Obj2Tiles.Library.Test.csproj	Copies new test data to output.
Obj2Gltf/WaveFront/ObjParser.cs	Parses extended vertex color tokens into ObjModel.Colors.
Obj2Gltf/WaveFront/ObjModel.cs	Adds storage for per-vertex colors.
Obj2Gltf/Converter.cs	Emits COLOR_0 and converts sRGB→linear when writing vertex data.
Obj2Gltf/BufferState.cs	Adds color buffer/accessor plumbing for glTF output.
.github/copilot-instructions.md	Updates project guidance to mention vertex color support and flow.