Two minor changes:
1) Structs are allowed to have matrix packing qualifiers
2) Adding an extra matrix packing qualifier shouldn't make
   a shader fail to compile

Change-Id: Icbe0178cb6017854c289db90349a17662e868095
Reviewed-on: https://swiftshader-review.googlesource.com/14228Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>