Validate pixel unpack buffer offset.

Validate pixel unpack buffer offset.

When a pixel unpack buffer is bound, glTexImage calls interpret the
<pixels> parameter as an offset into the pixel buffer. We weren't
validating that the accessed data falls within the buffer, when taking
the offset into account.

Bug chromium:822976

Change-Id: I3ab23e3b135fd4ad1e55555eec95d584684f5d82
Reviewed-on: https://swiftshader-review.googlesource.com/17928
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>