Many updates

Summary:
- Added new "Record GL app with FBO" activity.  It moves some
  simple shapes around the screen, and tries to use a GLES3
  feature when available.  Some basic infrastructure has been
  changed or introduced.
- Replaced TextureRender class with something that looks more
  like just rendering a big textured quad.
- Added summary of activities to README.
- Various cleanups and messups.