'get the width and height in display coords of the string s
with FontPropertry prop'

def get_text_width_height_descent(self, s, prop, ismath):

    if ismath:
        s = self.strip_math(s)
    if (self.gc is None):
        gc = self.new_gc()
    else:
        gc = self.gc
    gfx_ctx = gc.gfx_ctx
    font = self.get_wx_font(s, prop)
    gfx_ctx.SetFont(font, wx.BLACK)
    (w, h, descent, leading) = gfx_ctx.GetFullTextExtent(s)
    return (w, h, descent)
