FR: Add GL_MAJOR_VERSION and GL_MINOR_VERSION to the Missing Defs

To be added somewhere in JUCE/modules/juce_opengl/native/juce_MissingGLDefinitions.h:

   #ifndef GL_MAJOR_VERSION
    GL_MAJOR_VERSION = 0x821b,
   #endif

   #ifndef GL_MINOR_VERSION
    GL_MINOR_VERSION = 0x821c,
   #endif