Split gimp_transform_tool_get_active_item() into two functions:
gimp_transform_tool_get_active_item(), which returns the item
without checking for errors, and
gimp_transform_tool_check_active_item(), which returns the active
item while checking for errors.  Adapt the rest of the code to the
change.

Remove the invisible_layer_ok parameter of
gimp_transform_tool_check_active_item(), and always return an error
when the active layer is invisible.  This causes the flip and
measure tools to correctly reject invisible layers.  Un-hide the
active item in GimpTransformGridTool before transforming, to avoid
rejecting layers that were hidden by the tool.

(cherry picked from commit 360b25b9)