[cairo] Making _cairo_ft_scaled_font_get_load_flags() public

[Behdad Esfahbod]

In HarfBuzz's FreeType layer, I need to call FT_Load_Glyph myself.  When
someone uses both hb-ft and cairo-ft, I need the FT_Face load flags that cairo
uses to pass down to FT_Load_Glyph().  So, I'm suggesting that we make
_cairo_ft_scaled_font_get_load_flags() public.

There's some delicacy around VERTICAL_LAYOUT and the load_flags(), but that's
implementation details.

Comments?

behdad