Support for larger than "signed 32b limit" sized bitmaps in BGRAConvolve2D.

The multiplication of 2 signed ints was producing a result larger than what's supported by a single signed int and the memory was accessed out of bounds. Using uint64_t solves the issue.

BUG=378491
R=reed@google.com, rmistry@google.com

Author: sugoi@chromium.org

Review URL: https://codereview.chromium.org/323013005