Allow placing sprays at monuments

[WheteThunger]

Rust recently introduced spray decals which players can spray onto objects using the spray can. We should add the ability to add these to monuments.

Possible usage:

• maspray <skin id> (default spray is skin id is not specified)
• maspawn spray.decal + maskin <skin id>

Requirements:

• Sprays do not decay (server side or client side)
• Sprays cannot be washed away by splashing water
• Sprays can be resized with entity scale manager
• Spray icon can be changed with maskin <skin id> (might be tricky if the spray is attached to an entity, especially one that is spawned by the plugin)

Additional considerations:

• Free hand sprays
• Moving sprays with telekinesis

[WheteThunger]

I got something basic working. In the maspawn method (when aiming at a wall):

if (shortPrefabName == "spray.decal")
{
    localRotationAngles.x += 270;
    localRotationAngles.y -= 90;
}

Running maskin is not hitting the spray though, so if aiming at a spray that's on an entity spawned by this plugin, it will find that entity instead of finding the spray. That seems to be because it's on layer 1, and the raycast currently hits only Solid and Water. I was able to reskin to the 2818953599.

[WheteThunger]

Once OxideMod/Oxide.Rust#418 gets merged, we can place the following code in PostEntitySpawn().

var spray = Entity as SprayCanSpray;
if ((object)spray != null)
{
    spray.splashThreshold = int.MaxValue;
    spray.CancelInvoke(spray.RainCheck);
}

[WheteThunger]

The following commits have provided multiple improvements.

• Adjust rotation for initial spawn of spray.decaly entity
• Prevent spray decals being removed by rain or water splashes
• Disallow players from removing spray decals

For next steps, we can document how to add spray decals under "Instructions for specific entities", include the default spray skin ids, and include mention of how to create custom spray decals.

Freehand sprays, as well as moving sprays with Telekinesis (i.e., auto snap to surface while moving) will require additional research and are probably out of scope for this specific issue.