lv_font_get_glyph_dsc_fmt_txt() function
Used as `get_glyph_dsc` callback in lvgl's native font format if the font is uncompressed.
Arguments
dsc_out
store the result descriptor here
unicode_letter
a UNICODE letter code
unicode_letter_next
the unicode letter succeeding the letter under test
Return value
true: descriptor is successfully loaded into `dsc_out`. false: the letter was not found, no data is loaded to `dsc_out`