Restrict uniform array flattening to sampler and texture arrays.

Previously the uniform array flattening feature would trigger on loose
uniform arrays of any basic type (e.g, floats).  This PR restricts it
to sampler and texture arrays.  Other arrays would end up in their own
uniform block (anonymous or otherwise).  (Atomic counter arrays might be an
exception, but those are not currently flattened).

StandAlone/StandAlone.cpp

@@ -941,7 +941,7 @@ void usage()
            "                                          explicit bindings.\n"
            "  --amb                                   synonym for --auto-map-bindings\n"
            "\n"
-           "  --flatten-uniform-arrays                flatten uniform array references to scalars\n"
+           "  --flatten-uniform-arrays                flatten uniform texture & sampler arrays to scalars\n"
            "  --fua                                   synonym for --flatten-uniform-arrays\n"
            );