Fix -fsanitize=float-cast-overflow in Ogg/Theora writer

When compiled with Clang and mentioned flag, IO/Movie/vtkOggTheoraWriter.cxx
created thousands of out-of-range messages. The error was that the
brackets of the cast to unsigned char didn't enclose the whole
expression.

Change-Id: I885427f449ad3ced7b4484ea83dcf7887f2f33c7
Signed-off-by: Michael Wild <themiwi@gmail.com>
Merge topic 'fix-uchar-errors-in-oggtheora-writer' into master

9300708c Fix -fsanitize=float-cast-overflow in Ogg/Theora writer