Fix overflow sanitizer in EffectVisualizer.

This makes an implicit conversion to a signed integer explicit before
the unsigned integer overflow happens. This was causing runtime errors
on integer sanitized builds.

 runtime error: unsigned integer overflow: 1664 - 4096 cannot be
 represented in type 'unsigned int'

Bug: 30969751
Test: Compiles.
Change-Id: I1d4b5ab024e94c19ff448110064340125cd01e01
Merged-In: I1d4b5ab024e94c19ff448110064340125cd01e01