We can't sample from buffers with linear layout.

Bug swiftshader:104

Change-Id: I622786e9dac8faafeb7ef1745f02587bf43d67bd
Reviewed-on: https://swiftshader-review.googlesource.com/18168Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>