Implement lineOfSight on features

[aco4]

Features include a lineOfSight property:

warzone2100/data/base/stats/features.json

Lines 615 to 626 in f45d614

	"Tree1": {
	"armour": 10,
	"breadth": 1,
	"hitpoints": 50,
	"id": "Tree1",
	"lineOfSight": 0,
	"model": "mitrees.pie",
	"name": "*Tree1*",
	"tileDraw": 1,
	"type": "TREE",
	"width": 1
	},

It seems that this propety controls whether units and structures can see through the feature and shoot at objects behind it:

warzone2100/data/base/stats/features.ini

Lines 4 to 6 in 85b79c1

	; damageable <1|0> Whether or not the feature is damageable, if set to 0, the 'x' cursor will appear instead of the 'attack' cursor
	; lineOfSight <1|0> Line Of Sight. Whether of not units and structures can see through them and shoot at objects behind them
	; startVisible <1|0> Whether or not the feature is visible even if you haven't explored that area of terrain yet

However, this property doesn't seem to do anything in the source code currently. It seems that visibility.cpp is the right place to implement it?

warzone2100/src/visibility.cpp

Lines 1131 to 1154 in f45d614

	{
	const MAPTILE *psTile;
	halfway = current + (next - current) / 2;
	psTile = mapTile(map_coord(halfway.x), map_coord(halfway.y));
	if (TileHasStructure(psTile) && psTile->psObject != psTarget)
	{
	// check whether target was reached before tile's "half way" line
	part = halfway - start;
	partSq = dot(part, part);
	if (partSq >= distSq)
	{
	break;
	}
	// allowed to shoot over enemy structures if they are NOT the target
	if (partSq > 0)
	{
	angle_check(&angletan, oldPartSq,
	psTile->psObject->pos.z + establishTargetHeight(psTile->psObject) - pos.z,
	distSq, dest.z - pos.z, direct);
	}
	}
	}