Some tweaks to make this more friendly to my small machine.
Start introducing image reading.
Working version of raster frontend.
Currently the colour is fixed and the raster is represented by a rectangle and not raster data.
Render a crude output of the rastered image (from GCode).
Show the background and overscan locations in shades of grey.
Update to work with linked images as well as embedded.
Add support for inversion and X / Y Offsets in +/-'ve directions
Update to make frontend match backend (X or Y, not both)
Add PPI (Pulses Per Inch) Support to the frontend.
Increase TX buffer size to cope with fast, small path changes (e.g. spiral).
Note that you must update the firmware to support a 512 byte RX buffer.
Merge branch 'raster-dev'

Conflicts:
	backend/app.py
	backend/config.py
	backend/filereaders/__init__.py
	backend/filereaders/svg_reader.py
	backend/filereaders/svg_tag_reader.py
	frontend/app.html
	frontend/js/app.js
	frontend/js/app_gcodereader.js
	frontend/js/app_gcodewriter.js
	frontend/js/app_import.js
	frontend/js/app_laserjobs.js
	frontend/testCanvas.html