Option to activate PIXEL_SIZE_ON_SCREEN range mode for the mp driver + Fix for min LOD option

Hello,

I added support for the usage of osg::LOD:PIXEL_SIZE_ON_SCREEN range mode in the engine_mp driver.
This range mode works better if a orthographic projection is used to render the terrain. Additionally it seems to pick a more suitable LOD when the camera is positioned near the ground.
Further I fixed a bug that caused the min LOD option not to work correctly. With the bug fix the terrain will always be rendered with the minimum LOD or higher unless the specified LOD has not been loaded. In this case lower LODs will be used as a fallback till the minimum LOD is loaded.

Greetings

Marcel