add simple drawing functions for flickgame mega scripts

[iansheret]

This adds four new scripting functions to flickgame-mega, adding the capability to modify the images. The new functions are

• box: Draw a rectangle on the current image
• circle: Draw a filled circle on the current image
• line: Draw a line on the current image
• resetImage: Return the current image to its original state

One tricky design choice is indexing on the image coordinates, and also on colours. I've used zero-based indexes on both, as that's what the underlying data structures use, but it's maybe inconsistent with the numbering given to images (e.g. as used by the goto function).

[iansheret]

Now also adds overlayImage, which has selectable transparency as per our twitter discussion. Example game combining drawing and overlaying: https://www.dropbox.com/s/mkfolbo044mmgqm/lava.html?dl=0