osgEarth中opengl版本的确定

osgEarth/VirtualProgram
#if defined(OSG_GLES2_AVAILABLE)
#    define GLSL_VERSION                 100
#    define GLSL_VERSION_STR             "100"
#    define GLSL_DEFAULT_PRECISION_FLOAT "precision highp float;"
#elif defined(OSG_GLES3_AVAILABLE)
#    define GLSL_VERSION                 300
#    define GLSL_VERSION_STR             "300 es"
#    define GLSL_DEFAULT_PRECISION_FLOAT "precision highp float;"
#elif defined(OSG_GL3_AVAILABLE)
#    define GLSL_VERSION                 330
#    define GLSL_VERSION_STR             "330"
#    define GLSL_DEFAULT_PRECISION_FLOAT ""
#else
#    define GLSL_VERSION                 330
#    define GLSL_VERSION_STR             "330 compatibility" 
#    define GLSL_DEFAULT_PRECISION_FLOAT ""
#endif