Implement the deprecated gtk_snapshot_render api for carets with the new one.
Move the implementations from gtksnapshot.c to gtk/deprecated/gtkrender.c and deprecated these functions. We want to get rid of them. These functions are still used in some of our widgetry, so use G_GNUC_BEGIN/END_IGNORE_DEPRECATIONS around them.
These take a GtkStyleContext as argument, and we want to get rid of GtkStyleContext eventually. The proper drawing api these days is gtk_snapshot.