(Mdepth): Don't declare it here.

(Mdepth): Don't declare it here.
(draw_background): Call frame->driver->XXX instead of mwin__XXX.
(render_glyphs, render_glyph_string): Likewise.
(mdraw__init): Don't set Mdepth.
(mdraw_text, mdraw_image_text, mdraw_text_with_control): Call
M_CHECK_WRITABLE.
(mdraw_text_per_char_extents): Return 0 on success and -1 on
error.
(mdraw_text_items): Check FRAME is writable.
(mdraw_per_char_extents): Implement body.