Store files and objects as BMP files.
Recall BMP files as graphical objects.
Also adjust `Show` to not add a border to input graphical objects.
Fixes: #867
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
Make sure that `Undo` and `LastArg` work correctly for plotting
functions, integration and solver.
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
Add image checking from saved image references as a way to check that
the generated pixmaps are consistent from run to run.
Fixes: #732
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
