Add a "clip" property to GimpCavnasTransformGuides. When set, use
gimp_transform_polygon() for transforming the guides and the
bounding box, so that they're properly clipped.
Add a corresponding "clip-guides" property to
GimpToolTransformGrid, and set it to TRUE in GimpToolHandleGrid, so
that the handle-transform tool's guides are clipped properly.
... gimp_transform_matrix_generic()
Replace the separate x/y coordinate arrays of GimpHandleGrid with
GimpVector2 arrays, and use gimp_transform_matrix_generic(),
instead of gimp_transform_matrix_handles(), when calculating the
matrix. When the resulting matrix is invalid, hide the guides.
Rename gimp_tool_widget_snap_offsets() to set_snap_offsets(),
and add gimp_tool_widget_get_snap_offsets().
Also rename gimp_tool_widget_status() to set_status(), and
add new function and signal set_status_coords().
which is the code that calculates handle size based on pointer
proximity. Use the new function in GimpToolHandleGrid and
GimpToolLine, and clean up some stuff in GimpToolLine.
