Add perspective camera aspect ratio and zfar property flags

[jmousseau]

The perspective camera's aspectRatio and zfar are both optional. Previously, if you ran test_write on

{
    "asset": {
        "version": "2.0"
    },
    "cameras": [
        {
          "type": "perspective",
          "perspective": {
            "yfov": 0.5,
            "znear": 0.01
          }
        }
    ]
}

it would treat the aspect ratio and zfar both as zero

{
  "asset": {
    "version": "2.0"
  },
  "cameras": [
    {
      "type": "perspective",
      "perspective": {
        "aspectRatio": 0,
        "yfov": 0.5,
        "zfar": 0,
        "znear": 0.00999999978
      }
    }
  ]
}

With this change, test_write will produce

{
  "asset": {
    "version": "2.0"
  },
  "cameras": [
    {
      "type": "perspective",
      "perspective": {
        "yfov": 0.5,
        "znear": 0.00999999978
      }
    }
  ]
}