glsl: Only force varyings to be flat when varying packing.

glsl: Only force varyings to be flat when varying packing.

Varying packing would like to mark certain variables as flat.
This works as long as both sides of the interfaces are changed
accordingly.  However, with SSO, we disable varying packing on
the outermost stages.  We also disable varying packing for
certain tessellation stages.

With SSO, we operate on the producer and consumer separately.
Checks based on the consumer stage and variable are risky, and
can easily lead to altering one half of the interface between
stages, breaking SSO pipeline IO validation.

Just stop monkeying around with interpolation modes unless
required for varying packing.  There's no point.  This also
disables it in unsafe SSO cases.

Fixes CTS tests:
*.tessellation_shader.tessellation_control_to_tessellation_evaluation.gl_MaxPatchVertices_Position_PointSize

Also fixes Piglit's spec/oes_geometry_shader/sso_validation:
- user-defined-gs-input-not-in-block.shader_test
- user-defined-gs-input-in-block.shader_test

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>