now have function to get window style based on FreeGLUT's stateflag

now have function to get window style based on FreeGLUT's stateflag
(fgState.DisplayMode) so that window decoration specification is only in
one place in the code. Also, glutGet(GLUT_WINDOW_BORDER_WIDTH); and
glutGet(GLUT_WINDOW_HEADER_HEIGHT); now return the right sizes, i.e.,
taking the display mode into account, for the border and caption when no
window is created yet

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1396 7f0cb862-5218-0410-a997-914c9d46530a