To improve performance of image display in Firefly, allow an afwDisplay instance with the Firefly backend to track display.scale and display.pan settings that precede display.mtv, and apply these at the time of image display. Also add a small correction to the pan method so that it uses the LSST pixel coordinate convention.

Implemented:

Modify the display_firefly backend to remember the results of scale (stretch) commands, and to apply scale and pan parameters when displaying an image with lsst.afw.display.mtv.

• Remember and apply the serialized RangeValues string that Firefly uses
• Correct pan command by 0.5 pixels to follow LSST convention
• Apply the remembered pan position when using mtv
• Make 'slate.html' the default if it is not passed explicitly or if the environment variable FIREFLY_HTML is not defined. This is necessary for Firefly to show readout of LSST pixel convention.