Fix AssimpLoader collada texture coordinates (#634)

Fixes UNIT_AssimpLoader_TEST with assimp 5.4.3. This should fix homebrew CI.

This PR tweaks the way we checks for texture coordinates from assimp. Empty texture coordinate slots are now allowed as of assimp/assimp#5636. The new logic should be compatible with assimp 5.4.3 and prior versions

Signed-off-by: Ian Chen <ichen@openrobotics.org>

graphics/src/AssimpLoader.cc

@@ -650,14 +650,14 @@ SubMesh AssimpLoader::Implementation::CreateSubMesh(
     subMesh.AddVertex(vertex);
     subMesh.AddNormal(normal);
     // Iterate over sets of texture coordinates
-    int uvIdx = 0;
-    while(_assimpMesh->HasTextureCoords(uvIdx))
+    for (unsigned int i = 0; i < AI_MAX_NUMBER_OF_TEXTURECOORDS; ++i)
     {
+      if (!_assimpMesh->HasTextureCoords(i))
+        continue;
       math::Vector3d texcoords;
-      texcoords.X(_assimpMesh->mTextureCoords[uvIdx][vertexIdx].x);
-      texcoords.Y(_assimpMesh->mTextureCoords[uvIdx][vertexIdx].y);
-      subMesh.AddTexCoordBySet(texcoords.X(), texcoords.Y(), uvIdx);
-      ++uvIdx;
+      texcoords.X(_assimpMesh->mTextureCoords[i][vertexIdx].x);
+      texcoords.Y(_assimpMesh->mTextureCoords[i][vertexIdx].y);
+      subMesh.AddTexCoordBySet(texcoords.X(), texcoords.Y(), i);
     }
   }
   for (unsigned faceIdx = 0; faceIdx < _assimpMesh->mNumFaces; ++faceIdx)