Added comments about the danger of not clamping.

With clang's -fsanitize, several of VTK's tests
invoke undefined behaviour by converting doubles
to integer types that are out of range (ex: converting
12345.67 to uint8).  I have no fix, but these comments
will help anyone than runs into this.

Also improved debuggability and readability by adding
a temp local variable.

Change-Id: I0453b7bca693c416f7379f5b8c8cd60d0997937c
Merge topic 'sanitize-bounds' into master

107df717 Added comments about the danger of not clamping.