Clean up a 'void *' in a unix.h typedef.

'struct draw_ctx' has a structure tag inside gtkwin.c, so as per this
week's standard practice, let's expose the tag elsewhere so that
pointers declared that way can't be confused with anything else.

unix/unix.h

@@ -65,7 +65,7 @@ struct FontSpec {
 };
 struct FontSpec *fontspec_new(const char *name);
 
-typedef void *Context;                 /* FIXME: probably needs changing */
+typedef struct draw_ctx *Context;
 
 extern const struct Backend_vtable pty_backend;