A workaround of the dreaded Intel HD Graphics driver issue at least

on the laptop of @roesel.
fixes https://github.com/prusa3d/Slic3r/issues/672

The Intel HD Graphics hangs on the glFinish() call for some reason
with one particular graphics driver revision. Also the glFinish() call
was superfluous and it only may have had negative effect over the performance.

Both glFinish() and glFlush() were removed for performance reasons
where they were not needed, see
https://www.khronos.org/opengl/wiki/Common_Mistakes

lib/Slic3r/GUI/Plater/2DToolpaths.pm

@@ -338,7 +338,6 @@ sub Render {
     glClear(GL_COLOR_BUFFER_BIT);
     
     if (!$self->GetParent->enabled || !$self->layers) {
-        glFlush();
         $self->SwapBuffers;
         return;
     }
@@ -486,7 +485,6 @@ sub Render {
     }
     
     gluDeleteTess($tess) if $tess;
-    glFlush();
     $self->SwapBuffers;
 }