Last commit reintroduced this bug.

Allow transforming invisible layers using transform-grid tools, by
adding a 'drawable' member to GimpTransformTool, and setting/
clearing it when initializing/halting a GimpTransformTool.  In
gimp_transform_tool_check_active_item(), skip the visibility check
if the active item equals the GimpTransformTool's 'drawable'
member.

(cherry picked from commit cea6f1dc)