Export data for underline

Based on https://github.com/lvgl/lvgl/issues/1618#issuecomment-651403534

FreeType can tell where to place the underline of a text and how thick it should be. See here.

It’d be great to add this info to the font descriptors like this:

/*Initialize a public general font descriptor*/
lv_font_t lv_font_montserrat_12 = {
    .get_glyph_dsc = lv_font_get_glyph_dsc_fmt_txt,    /*Function pointer to get glyph's data*/
    .get_glyph_bitmap = lv_font_get_bitmap_fmt_txt,    /*Function pointer to get glyph's bitmap*/
    .line_height = 15,          /*The maximum line height required by the font*/
    .base_line = 3,             /*Baseline measured from the bottom of the line*/
#if !(LVGL_VERSION_MAJOR == 6 && LVGL_VERSION_MINOR == 0)
    .subpx = LV_FONT_SUBPX_NONE,
#endif
#if LV_VERSION_CHECK(7, 2, 0)
    .underline_ofs = 3,
    .underline_tickness = 2,
#endif
    .dsc = &font_dsc           /*The custom font data. Will be accessed by `get_glyph_bitmap/dsc` */
};

[puzrin]

released 1.1.0

[puzrin]

How does this work when fonts are mixed? Do we just use the values from the first font (like the other metrics)?

Yes, metrics of first font will be prefered. The same for ascent/descent and so on.

Font mixing is to create new fonts from icons, or to add some icons into font. If anyone wish to merge different text fonts - good luck 😃