Fix out-of-bounds read unpacking sparse accessor values with byteStride

[SAY-5]

cgltf_accessor_unpack_floats advances through the sparse values buffer view using accessor->stride, but the sparse values are tightly packed, so cgltf_validate only sizes that buffer view as element_size * sparse->count. When the accessor's byteStride is larger than the packed element size, the second pass reads past the end of the values buffer view even though the file passes validation. This walks the sparse values by the packed element size instead, matching what validate checks. Fixes #295.