Add simple tile visibility checking in the ViewModule

A small thing. 
Its always recommended to do multiplication over division. Sometime the performance gains are huge.. 
Some Floating point processing multiplication can be 5 to 8 times faster than floating time division.

float inversePowZoon = 1/pow(2, m_zoom);
float tileSize = 2*PI*EARTH_RADIUS_M*inversePowZoon;