MDL-43462 assignfeedback_editpdf: fix display of landscape PDFs

The size of the drawing canvas is now set from the size of the generated page image, with automatic
scrolling if it is too wide to display
Absolutely-positioned nodes (comments + stamps) are repositioned as the drawing canvas is scrolled

Conflicts:
	mod/assign/feedback/editpdf/yui/build/moodle-assignfeedback_editpdf-editor/moodle-assignfeedback_editpdf-editor-min.js