layers: Add NULL/sType checks for struct members

layers: Add NULL/sType checks for struct members

Add param_checker support for validating struct members.  Messages
are logged for the following conditions, based on conditions
specified in vk.xml:
 - A pointer is NULL and is not marked as optional in the XML
 - An array is NULL and is not marked as optional in the XML, unless
   its count is 0
 - An array count is 0 and is not marked as optional in the XML
 - A structure's sType value does not match the value specified
   in the XML

Addresses GL105, GL109, GH82

Change-Id: I7063fe2582b30fdfc0006fe945a0f9c84a2aa66a