[AD] line drawing to memory bitmaps

[Peter Wang <novalazy@xxxxxxxxxx>]

Hi,

I just added a line drawing example.  If you define TEST_MEMBMP it
crashes when trying to use al_draw_line() to draw to a memory bitmap.
I'll let someone else debug it.  At a guess, it seems not to be taking
into account the lock region offset (?)

Also, did I use ALLEGRO_SINGLEBUFFER correctly?

Peter