font: pango: fix a rendering problem of certain CJK fonts

This patch fixes a rendering problem of certain CJK fonts which causes
the rendered CJK characters to appear downward-shifted.

For some CJK fonts, pango_layout_get_pixel_extent() returns different
logical_rect for CJK glyphs and Latin glyphs. Therefore, using -rec.y
as the y position of the baseline will render the CJK glyphs downward-
shifted. face.baseline will be a better choice.