This commit enhances the output transform
command with options for a relative transform,
i.e. the provided transform will be applied as
an offset to the current transform. Append
`clockwise` to rotate clockwise from the current
rotation, or `anticlockwise` to rotate in the
opposite direction.
For example, if the output LVDS-1 is rotated
90 degrees clockwise, the command
`output LVDS-1 transform 90 clockwise`
will rotate the display to 180 degrees.
All transform options are supported,
including flipped transforms.
Relative transforms can only be applied to
a single output and cannot be used with
a wildcard (*) output specifier.
