Brush tool enhancements
Detail: A visual brush 'palette' now makes it easy to see the internal shapes or chosen sprite. More clear brush tool window layout. Adds an opacity slider. Tint of a sprite using the currently selected colour using a ColourTrans transfer function. Use alpha channel sprite as the brush sprite. Fix to colours_entry() in Colours.c for 16M colour sprites.
Notes - The non-100% opacity features are disabled if supporting modules are missing/too old. Translucent painting is performed into an off screen buffer, then the result merged onto the sprite once the mouse is idle. Admin: Submission for Paint bounty.