Fixed GLSL error in Ground_Scattering_Fragment shader - due to not impli...

...cit conversion of vec4 to vec3:

<log output>
FRAGMENT glCompileShader "atmos_fragment_main" FAILED
FRAGMENT Shader "atmos_fragment_main" infolog:
Fragment shader failed to compile with the following errors:
WARNING: 0:33: warning(#402) Implicit truncation of vector from size: 4 to size: 3
ERROR: 0:33: error(#162) Wrong operand types: no operation "+" exists that takes a left-hand operand of type "uniform 4-component
vector of vec4" and a right operand of type "highp 3-component vector of vec3" (or there is no acceptable conversion)
WARNING: 0:33: warning(#402) Implicit truncation of vector from size: 4 to size: 3
ERROR: error(#273) 1 compilation errors.  No code generated
</log>