In GimpCanvasTransformPreview, use a GimpPickable, instead of a
GimpDrawable, as the preview source, so that we can use it with the
image projection, rather than just with drawables.
(cherry picked from commit 6023f97509)
In GimpCanvasTransformPreview, use the transform matrix to
determine if we're doing a perspective transform, rather than
relying on a separate property, so that we don't use the slow
perspective path unnecessarily.
Consequently, remove the does_perspective member of
GimpTransformTool, since it's no longer used.
Instead, add drawable, transform, x1, y1 etc. properties. This is
cleaner and has the nice side effect of not leaving artifacts, because
the changed state of the transform tool doesn't affect the extents
calculation any longer.
And remove all the complicated handling code entirely. This makes
GimpTransformTool a lot less complex. As a nice side effect, the
preview is now always 100% in sync with the grid and handles.
