app: fix curves tool color picking
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.
Showing with 21 additions and 28 deletions