The hack to keep the blueprints visible broke other interactions. Add a more
targetted hack to make it work. Also fix willy's head flipping when it shouldn't.
There are still some issues overall with head rendering (lipsync issues) but
it's much better now I think.
This fixes#15821.
Willy Beamish should pause script execution while waiting for a response in a
dialog, but not in dialogs with no head.
Heads should also have an "opening" state for one frame before they are
considered visible. This avoids a frame of overlap with the previous one, just
like dialogs do. Without this, eg, the background is not rendered behind the
teacher after returning from the principal's office during the opening
sequence.
These fixes bring Willy Beamish rendering very close to the original.
