[AD] bitmaps flags in ttf addon

[Peter Wang <novalazy@xxxxxxxxxx>]

I think the call to get bitmap flags in render_glyph in the TTF addon
(to check for ALLEGRO_NO_PREMULTIPLIED_ALPHA) is too late.  The user
should not expect that the new bitmap flags have an effect
when drawing a string.

I suggest we cache the premultiple alpha bit when the font is loaded,
unless there is a better idea?

Peter