bitmap_top
The bitmap's top bearing expressed in integer pixels.
Remember that this is the distance from the baseline to the top-most glyph scanline,
upwards y coordinates being positive.
位图字符从 baseline 到位图字符顶端的高度。注:baseline 在 y = 0 的位置。某些字符的最低位置可能小于0,例如: g
bitmap_left
The bitmap's left bearing expressed in integer pixels. Only valid if the format is FT_GLYPH_FORMAT_BITMAP,
this is, if the glyph slot contains a bitmap.