GenericCAO: Fix dark model below y = 0

Move point at which light is sampled up to 0.5 nodes above foot level,
to avoid that point sometimes passing into the node below causing the
model to go dark.

src/content_cao.cpp

@@ -946,7 +946,7 @@ void GenericCAO::updateLightNoCheck(u8 light_at_pos)
 
 v3s16 GenericCAO::getLightPosition()
 {
-	return floatToInt(m_position, BS);
+	return floatToInt(m_position + v3f(0, 0.5 * BS, 0), BS);
 }
 
 void GenericCAO::updateNodePos()