glsl: Distinguish between no interpolation qualifier and 'smooth'

glsl: Distinguish between no interpolation qualifier and 'smooth'

Previously, we treated the 'smooth' qualifier as equivalent to no
qualifier at all.  However, this is incorrect for the built-in color
variables (gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, and
gl_BackSecondaryColor).  For those variables, if there is no qualifier
at all, interpolation should be flat if the shade model is GL_FLAT,
and smooth if the shade model is GL_SMOOTH.

To make this possible, I added a new value to the
glsl_interp_qualifier enum, INTERP_QUALIFIER_NONE.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>