utils: fix build with <GL/glext.h> version >= 85.

utils: fix build with <GL/glext.h> version >= 85.

Mesa recently updated the <GL/glext.h> header version to Khronos version 85.
This caused the PFNGLMULTITEXCOORD2FPROC definition to be moved out of the
GL_VERSION_1_3_DEPRECATED block. However, since <GL/gl.h> also defines
GL_VERSION_1_3 to 1, the definitions in <GL/glext.h> are then not enabled,
thus leaving PFNGLMULTITEXCOORD2FPROC undefined as well.

Provide a PFNGLMULTITEXCOORD2FPROC replacement as an interim solution for
newer versions of the <GL/glext.h> header.