opengl: adjust rect drawing - should be to width (height) - 1

Might be the cause of Rectangle implementation of GetWidth and
GetHeight which is different to other programs.

X11 backend compensates in the same way for this use case.

Change-Id: Ibc3c1d6f442d616c64b602cccb601bfc33fd4baf
(cherry picked from commit a1385d37)
Reviewed-on: https://gerrit.libreoffice.org/16918Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/opengl/gdiimpl.cxx

@@ -1197,8 +1197,8 @@ void OpenGLSalGraphicsImpl::drawRect( long nX, long nY, long nWidth, long nHeigh
     {
         GLfloat fX1 = OPENGL_COORD_X(nX);
         GLfloat fY1 = OPENGL_COORD_Y(nY);
-        GLfloat fX2 = OPENGL_COORD_X(nX + nWidth);
-        GLfloat fY2 = OPENGL_COORD_Y(nY + nHeight);
+        GLfloat fX2 = OPENGL_COORD_X(nX + nWidth - 1);
+        GLfloat fY2 = OPENGL_COORD_Y(nY + nHeight - 1);
 
         GLfloat pPoints[16];