fix(gltf): add HAS_COLORS define for COLOR_0 vertex attribute

[nokonoko1203]

Background

When a glTF mesh contains a COLOR_0 vertex attribute (e.g. point cloud data with per-vertex colors), parsePBRMaterial() did not set the HAS_COLORS shader define. The existing code already handled analogous attributes using the same #ifdef pattern.

if (attributes['NORMAL'])     parsedMaterial.defines['HAS_NORMALS'] = true;
if (attributes['TANGENT'] ..) parsedMaterial.defines['HAS_TANGENTS'] = true;
if (attributes['TEXCOORD_0']) parsedMaterial.defines['HAS_UV'] = true;
// COLOR_0 was missing

Without HAS_COLORS, the vertex shader's #ifdef HAS_COLORS block was excluded at compile time, so the in vec3 colors declaration was absent. The COLOR_0 data was uploaded to the GPU but never read by the shader — all vertices were rendered with the default instance color (white).

Downstream consumers such as deck.gl's ScenegraphLayer do not currently handle per-vertex colors from glTF meshes, but would need this define to conditionally declare vertex attribute inputs and blend per-vertex colors with instance colors (e.g. for rendering colored point clouds via 3D Tiles).
The Khronos https://github.com/KhronosGroup/glTF-Sample-Renderer also uses the same compile-time define pattern (HAS_COLOR_0_VEC3 / HAS_COLOR_0_VEC4) for vertex color support.

Change List

• Add HAS_COLORS define when COLOR_0 attribute is present in parsePBRMaterial() function
• Add equivalent in the commented-out PBRMaterialParser class for consistency