sdgui: implement text selection
gdk_pango_layout_get_clip_region() can be used to retrieve clip regions for text selection. Other functions exist to retrieve what position corresponds to display coordinates.
pango_cairo_layout_path() can be used to override the foreground colour (Pango markup may specify colours of its own, even if it's not currently supported; besides, I believe
pango_cairo_show_layout() will employ some optimisations).
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