A subclass of GimpTransformTool, to be used as a base class for
transform tools that calculate their matrix based on 4 pairs of
input/output points, and that display the transform matrix as their
GUI (this includes the unified, perspective, and handle transform
tools; the next commit ports them over to
GimpGenericTransformTool).

When the resulting matrix of the input/output mapping sends any of
the output points to, or past, infinity, GimpGenericTransformTool
sets GimpTransformTool::transform_valid to FALSE, and displays an
appropriate message in the tool GUI, instead of showing the matrix.