Use adhoc function to set GL_GENERATE_MIPMAP to true, and fix minor things

Use adhoc function to set GL_GENERATE_MIPMAP to true, and fix minor things

Under windows lablGL is most likely built with ancient 1.1 OpenGL headers
which do not define GL_GENERATE_MIPMAP and any attempt to use it results
in a uncaught exception.

Remove extra semicolon and use Glut to query extension instead of GlMisc
which is not available in any released version of LablGL.