avoid unnecessary draw

src/Frame.cc

 #ifdef OSMESA
 #  define GL_GLEXT_PROTOTYPES
 #  include <GL/osmesa.h>
+#  define USE_FBO           // antialiasing
 #elif  defined(__APPLE__)
 #  define GL_SILENCE_DEPRECATION
 #  include <OpenGL/gl3.h> 
@@ -10,11 +11,10 @@
 #  endif
 #  include <GL/glew.h>
 static bool initglew = false;
+#  define USE_FBO        // antialiasing
+//#  define USE_FLTK_IMG
 #endif
 
-#define USE_FBO        // antialiasing
-//#define USE_FLTK_IMG
-
 #include "Frame.h"
 
 /**
@@ -96,11 +96,6 @@ Frame :: Frame( TBmeshWin *t, int w, int h, bool fa ) :_tbwm(t),_w(w),_h(h),
 #ifdef USE_FBO
   make_framebuffer( w, h, _fbms, _color_rbms, _depth_rbms, true );
   make_framebuffer( w, h, _fb,   _color_rb,   _depth_rb );
-#endif
-
-  fill_buffer( w, h, true );
-
-#if defined(USE_FBO)
   glBindFramebuffer(GL_FRAMEBUFFER, 0); // restore the display
 #endif //OSMESA
 }
@@ -122,8 +117,8 @@ Frame :: ~Frame()
 void 
 Frame :: delete_objs()
 {
+  if (_buffer) delete[] _buffer;
 #if defined(USE_FBO)
-  delete[] _buffer;
   glDeleteRenderbuffers(1, &_color_rb);
   glDeleteRenderbuffers(1, &_depth_rb);
   glDeleteFramebuffers(1, &_fb);