Added proper validation to make sure the pixel unpack buffer
is both unmapped and large enough, when imageSize is specified.

Change-Id: If6ec764d741bb9d63d38d0656188846c5a9be66d
Reviewed-on: https://swiftshader-review.googlesource.com/13868Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>