mesa: Add an option so driver can opt out of varying packing.

mesa: Add an option so driver can opt out of varying packing.

On hardware that supports a limited number of texture indirections,
varying packing will comsume an extra texture indirection, since ALU
operations are needed in the fragment shader to unpack the varyings
before any texturing can be done.

This patch introduces a new driver option,
ctx->Const.DisableVaryingPacking, which can be used by a driver to opt
out of varying packing if the extra texture indirection is costly
enough to outweigh the advantages of packing varyings.

Reviewed-by: Marek Olšák <maraeo@gmail.com>