Improved water rendering

* fixed visible border between ocean and local water
* lily pads now rendered as separate entities
* lily pads now cast shadows
* lily pads are now visible from below
* cleaned up water rendering code
* cleaned up Octree intersection code
* improved water rendering performance
* added new "Water" tab to Render Controls
* added water visibility slider
* added water opacity slider
* removed "clear water" option (obsoleted by visibility slider)
* warn when loading a scene saved with a newer SDF version
* bumped SDF version

fixes #166 (github)
fixes #255 (github)
fixes #286 (github)