glsl: set user defined varyings to smooth by default in ES

glsl: set user defined varyings to smooth by default in ES

This is usually handled by the backends in order to handle the
various interactions with the gl_*Color built-ins.

The problem is this means linking will fail if one side on the
interface adds the smooth qualifier to the varying and the other
side just uses the default even though they match.

This fixes various deqp tests. The spec is not clear what to for
desktop GL so leave it as is for now.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92743