docs: add ARB_sample_shading to GL3, update relnotes

relnotes weren't updated this whole time, so I went through all the GL3
changes and picked out the nouveau ones since 10.1.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0/ir: do constant folding of extbf/insbf

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50/ir: change texture offsets to reference values, allow nonconst

This allows us to have non-constant offsets for textureGatherOffset and
textureGatherOffsets.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>