Use cairo_scaled_font_t object for text drawing and metrics calculation

Use cairo_scaled_font_t object for text drawing and metrics calculation

* src/ftfont.h (struct font_info): Replace member cr_font_face of type
cairo_font_face_t * with cr_scaled_font of type cairo_scaled_font_t *.
* src/ftcrfont.c: Include math.h for floor, ceiling, and lround.
(ftcrfont_glyph_extents): Use cairo_scaled_font_glyph_extents.
(ftcrfont_open): Create cairo_scaled_font_t object and set it to
cr_scaled_font member of struct font_info.
(ftcrfont_close): Use cairo_scaled_font_destroy.
(ftcrfont_draw): Use cairo_set_scaled_font.