Get the extents of a glyph specified by its character code.
The rectangle describes the the smallest rectangle containing all pixels that are touched when drawing the glyph. It is reported relative to the baseline. If you only need the advance, pass NULL for the rectangle.
当绘制图象时，黑色的矩形描绘最小的矩形被创建，包含了所有象素。它以基线为准被报告。如果你只需要advance，传递NULL为矩形。
The advance describes the horizontal offset to the next glyph (without kerning applied). It may be a negative value indicating left-to-right rendering. If you don't need this value, pass NULL for advance.
advance描绘水平偏移到下一个图像供给制（没有字距调整值）。它可能是一个负值，表明是从左到右绘制。如果你不需要这个值，传递NULL为advance。
