Fixed all clang -Wimplicit-fallthrough warnings:
- Some fixed by adding missing breaks
- Other fixed by adding VTK_FALLTHROUGH statement

Under (modern) clang with C++11:
 #define VTK_FALLTHROUGH [[clang::fallthrough]]

Otherwise:
 #define VTK_FALLTHROUGH ((void)0)

VTK_FALLTHROUGH serves two purposes:
 - under clang, suppresses -Wimplicit-fallthrough warnings
 - makes code more self-documenting