glsl: Round struct size up to at least 16 bytes

glsl: Round struct size up to at least 16 bytes

Per rule #9, the size of the structure is vec4 aligned.  The MAX2 in the
loop ensures that sizes >= 16 bytes are vec4 aligned.  The new MAX2
after the loop ensures that sizes < 16 bytes are vec4 aligned.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82932
Cc: mesa-stable@lists.freedesktop.org