Since commit bc458996, GimpFilterTool
assumes that color picking is only ever enabled through color picker
toggles, created using gimp_filter_tool_add_color_picker().  However,
the curves tool enables color picking using the color tool functions
directly.  CRITICALS ensued.

Use the new gimp_filter_tool_enable_color_picking() function, added
in the previous commit, and handle picking through the filter tool
interface, instead of the color tool interface.  This fixes the
issue.