Extracted some common code to a subroutine, in glutMainLoopEvent().

(The common code was a snippet to compute X keyboard modifiers as a
bit-mask of GLUT symbols.)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@288 7f0cb862-5218-0410-a997-914c9d46530a