Speed up opening fonts on Haiku

Speed up opening fonts on Haiku

* src/font.h (font_property_index): Note that some font drivers
use the extra data in a font entity to store driver-specific
information.

* src/haiku_font_support.cc (BFont_find): Set font indices.
(be_open_font_at_index): New function.
(BFont_open_pattern): Clean up coding style.
* src/haiku_support.h (enum haiku_font_specification)
(struct haiku_font_pattern): New fields and specifications for
indices.
* src/haikufont.c (haikufont_pattern_to_entity, haikufont_open):
Use indices to open fonts if available in the extra data.