Made vtkFrameBufferObject be built for ES 2 as
ES 2 does support it. This required a few changes.

Made a number of fixes to the Android/IOS super build process. Fixed
the install prefix to be pushed down to the subbuilds. Made a number
of top level setting cached in the sub builds because they are cached
there already and need to be overriden in the superbuild.  Otherwise
top level changes did not propagate down.