Add fence polygons

[Ryanf55]

Demo

fence-items.txt

Example Inclusion Triangle

Here's some sample MAVlink data for an inclusion triangle.

{
    "0": {
        "mavpackettype": "MISSION_ITEM",
        "target_system": 255,
        "target_component": 230,
        "frame": 0,
        "command": 5001,
        "current": 0,
        "autocontinue": 0,
        "param1": 3,
        "param2": 0,
        "param3": 0,
        "param4": 0,
        "x": -35.3617457,
        "y": 149.15790859999998,
        "z": 0,
        "mission_type": 1
    },
    "1": {
        "mavpackettype": "MISSION_ITEM",
        "target_system": 255,
        "target_component": 230,
        "frame": 0,
        "command": 5001,
        "current": 0,
        "autocontinue": 0,
        "param1": 3,
        "param2": 0,
        "param3": 0,
        "param4": 0,
        "x": -35.3594547,
        "y": 149.160323,
        "z": 0,
        "mission_type": 1
    },
    "2": {
        "mavpackettype": "MISSION_ITEM",
        "target_system": 255,
        "target_component": 230,
        "frame": 0,
        "command": 5001,
        "current": 0,
        "autocontinue": 0,
        "param1": 3,
        "param2": 0,
        "param3": 0,
        "param4": 0,
        "x": -35.3617457,
        "y": 149.1617503,
        "z": 0,
        "mission_type": 1
    }
}

https://sandcastle.cesium.com/#c=bZHfT8IwEMf/lcueRjI7cEx+DaIB30w0aHxxPpTtgMauJdcOMgz/u2WDaJC2ae/az/fucs20Mha2AndIMAaFO5iiEWXB3us7P/Wy2p9qZblQSKnXGqUqVVmtJMxftKxWWjl1E4ahssIKNIznuf+dKgDFCxxC6s0xh80Jd8uUtOQZpl5whE4PQ6glAGuBxClbV8NzSVNO1llcRWxJupjhihDNAxGv/I9GBHA+O90B69x1enE7CuAmilnknG7cCy6IdnR7BuJBN+5eAnFv0O7Hg3r0r0b6bAWNUXCLJLj8LVhLTWz+OKuBg9sPTfOaeerXXuviTfsX3WuNwAu8xNhK4uRc0b0oNposlCR9xkKLxUa6pCZclNkXWpYZc0xwRJPwrzTJxRZEPr7ynZBJbox7WZZSvoq9+49JEjr+n1Rqngu1et4iSV4dsXVn8tRcMsaS0LnXlVZrueB0EfkH

Details

Add parsers for inclusion and exclusion fences. These aren't super robust to dropped data yet, but they work in the happy path.
Fence clear still works.

A parser builds up an array of MISSION_ITEM objects that are for the fence, then once the expected count is reached, it calls the respective draw function.

Lots of code duplication which I can refactor later.