3D texture sampling did not support width*height > MAX_USHORT(65535).
Changed for 32 bit integer computation for that multiplication to
solve this issue.

Verified with 3D texture related dEQP tests.

Bug b/26105892 chromium:763382

Change-Id: Ifd07278f6263d000620a0b525c07ed4fcbdcf2e7
Reviewed-on: https://swiftshader-review.googlesource.com/12988Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>