ScanLine: No need to free the list of active edges

The list of active edges (ael) should be empty by the time the polygon
is completely filled. So there is no need to explicitely free it, the
execution can never enter the loop body.

Signed-off-by: Celelibi <celelibi@gmail.com>
ScanLine: outline the code that scan one line

It's a bit more clean when the code that scan only one line is separated
from the rest of the code.

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