Pixel unpack buffer validation follow up

Pixel unpack buffer validation follow up

2 fixes:
- The offset check was removed, as it only affects the pointer
  and not the size
- The modulo check is on the type only and not the entire image size

Change-Id: I8c4b64e845b2fae61959d7c62d2c5dc222249c68
Reviewed-on: https://swiftshader-review.googlesource.com/14009
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>