Support tslib-style touchscreen calibration.
We might use tslib itself to provide this support.

We should support at least:
 - Reflection (when the X or Y axis goes in the opposite direction to what we expect)
 - Rotation in 90-degree increments
 - Bounds adjustment

This might be done with five parameters: minX, minY, maxX, maxY, swapXY
or with a 2D matrix transform
or by reading touch input from tslib - which would give us access to other features of tslib such as dejitter. However, we might not always be able to rely on tslib being available.