Added skin box hide bit functionality

Added code to hide skin boxes when wearing a helmet if bit for said functionality is present.

Minecraft.Client/ModelPart.cpp

@@ -13,6 +13,7 @@ void ModelPart::_init()
 	compiled=false;
 	bMirror = false;
 	visible = true;
+	hideWithHelmet = false;
 	neverRender = false;
 	x=y=z = 0.0f;
 	xRot=yRot=zRot = 0.0f;