gallium: Force blend color to 16-byte alignment

gallium: Force blend color to 16-byte alignment

This aligns the 4-element color float array to 16 byte boundaries.  This
should allow compiler vectorizers to generate better optimizations.
Also fixes broken vectorization generated by Intel compiler.

v2: Fixed indentation and added a lengthy comment explaining the
    reason for the alignment.

Cc: <mesa-stable@lists.freedesktop.org>
Reported-by: Tim Rowley <timothy.o.rowley@intel.com>
Tested-by: Tim Rowley <timothy.o.rowley@intel.com>
Signed-off-by: Chuck Atkins <chuck.atkins@kitware.com>
Acked-by: Roland Scheidegger <sroland@vmware.com>