glsl: don't sort varying in separate shader mode

glsl: don't sort varying in separate shader mode

This fixes an issue where the addition of the FLAT qualifier in
varying_matches::record() can break the expected varying order.

It also avoids a future issue with the relaxing of interpolation
qualifier matching constraints in GLSL 4.50.

V2: (by Timothy Arceri)
* reworked comment slightly

Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>