Ignore earth_nmap.dds

Updated gitignore to not always request to commit the earth
normal/height-map file which is too large for github.
Transparent atmosphere mesh

Updated the sky mesh to integrate properly with the vertex and index
buffers (and their offsets).  Also added a render state configuration to
enable alpha-based blending when rendering the sky mesh.
Working on implementing ONeil's shaders

For realistic atmospheric scattering of light.