Toggle the filling indicator

Toggle is_filled boolean as needed. Especially, it is set to false when
the polygon is reopened and never set to true when the polygon is
already open.

Signed-off-by: Celelibi <celelibi@gmail.com>
Corrected a few warnings

Some function arguments are mandatory because they are OpenGL callback
functions. Now they won't be warned by gcc.

Signed-off-by: Celelibi <celelibi@gmail.com>
polygon_get_last_vertex wasn't used

This function was declared static and wasn't used at all. If needed,
it's still in the git history.

Signed-off-by: Celelibi <celelibi@gmail.com>
Ppm.c: Commented out useless function

There were two static functions in Ppm.c that weren't used and produced
warnings.

Signed-off-by: Celelibi <celelibi@gmail.com>