+ Calculate the visible box for displaying glyphsrc in dest.
+
+ display_box and display_glyph_area are used to represent an area to
+ displayed and where to display it. Using these two structures all
+ combinations of clipping and position can be accommodated.
+
+ dest - display_box
+
+ xpos - absolute horizontal position of area.
+
+ ypos - absolute vertical position of area.
+
+ glyphsrc - display_glyph_area
+
+ xoffset - horizontal offset of the glyph, +ve means display
+ the glyph with the x position offset by xoffset, -ve means
+ display starting xoffset into the glyph.
+
+ yoffset - vertical offset of the glyph, +ve means display the
+ glyph with y position offset by yoffset, -ve means display
+ starting xoffset into the glyph.
+