[AD] windows port crashes due to (wrong?) asserts

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

I have reports of my windows A5 .exe failing with this:

"Assertion failed: dm.dmFields & 
0x00100000, file /home/elias/prog/allegro/git/src/win/wgl_disp.c, line 
1620"

The relevant lines are:

1620   ASSERT(dm.dmFields & DM_PELSHEIGHT);
1621   ASSERT(dm.dmFields & DM_PELSWIDTH);
1622   ASSERT(dm.dmFields & DM_POSITION);

Why are those asserts? Seems perfectly valid to me if Windows doesn't like reporting all of them each time...