[AD] load_font

[Evert Glebbeek <eglebbk@xxxxxxxxxx>]

Attached is a patch that adds the often-requested load_font function. The 
patch defines the following functions:

void register_font_file_type (AL_CONST char *ext, FONT *(*load)(AL_CONST 
char *filename, RGB *pal, void *param))
FONT *load_font (AL_CONST char *filename, RGB *pal, void *param)

These work similar to the normal register_bitmap_file_type/load_bitmap 
functions. I have added an extra parameter param to the load_font routine 
in case a specific loader wants to do anything with this. Only the dat 
loader actually does anything with this currently.

FONT *load_dat_font(AL_CONST char *filename, RGB *pal, void *param))
FONT *load_bios_font(AL_CONST char *filename, RGB *pal, void *param))
FONT *load_grx_font(AL_CONST char *filename, RGB *pal, void *param))
FONT *load_grx_or_bios_font(AL_CONST char *filename, RGB *pal,void *param))
FONT *load_bitmap_font(AL_CONST char *fname, RGB *pal, void *param))

These load fonts from specific types of files. Most of the code was 
directly ripped from the dat/grabber plugin. The load_bitmap_font() code 
is special in that load_font() calls this to load a font if it doesn't 
know the font type. It calls load_bitmap() internally, so it will know how 
to load a font from any registered bitmap type without having to be 
installed for it.
With this code pulled from the datafile plugin into Allegro, the code from 
the plugins can be (and should be) significantly simplified. I haven't 
done this yet, nor written documentation for this.
Tested somewhat in Linux.

Evert