WaveFrontReader: read per-vertex colors from "v x y z r g b"

[Joel Kiptoo (Kiptoo-Deus)]

Fixes #75.

Tools such as MeshLab write a per-vertex color after the position on v lines (v x y z r g b). The reader read x y z and dropped the rest of the line, so these colors were lost.

Changes in WaveFrontReader.h:

• On a v line, the rest of the line is parsed after x y z. If there are exactly three extra values they are read as an RGB color. A single extra value is still the optional w coordinate from the OBJ spec and is ignored, and any other count is ignored as well. Positions without a color default to white.
• The colors go into a new public vertexColors array (XMFLOAT3, one entry per entry in vertices) with a hasVertexColors flag, filled in as vertices are created from faces. When the file has no colors the array stays empty.
• Vertex itself is unchanged, so the .vbo layout used by LoadVBO and existing callers are not affected.
• The rest of the line is read into a std::wstring rather than a fixed buffer, so an overlong v line can't put the stream into a failed state.

Vertex deduplication is keyed on the position index, so vertices that share a position also share its color and the existing dedup behaviour doesn't change.

Testing: on macOS with clang, using the same small shim as in #324 (std::wifstream from a wchar_t* path is an MSVC extension libc++ doesn't have), I loaded:

• the MeshLab snippet from the issue: colors 0.984314 0.764706 1.0 etc. come through with the right positions and normals
• a quad with a different color on each corner, shared between two faces: 4 vertices, 4 colors
• a file without colors and a file using v x y z w: hasVertexColors is false and vertexColors is empty
• a file where only some vertices have a color: the others are white
• CRLF line endings, an end-of-line comment and relative (negative) indices
• a v line with 1,700 extra values: the rest of the file still loads

I didn't touch meshconvert. If you'd like the colors passed through to its output formats I can follow up on that separately.

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