nv50: don't forget to also clear additional layers

Fixes most of the tests/spec/gl-3.2/layered-rendering/* piglits.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nvc0: don't forget to also clear additional layers

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50: copy nvc0's get_sample_position implementation

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50/ir: un-ms samplers
nv50: report glsl 1.50 now that gp tests pass

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50: enable seamless cube maps/multisample textures for OpenGL 3.2

Some of the hardware support is missing. The NVIDIA-provided driver,
which claims 3.3 support fails a slew of the relevant tests as well.
This allows us to expose geometry shaders without doing the additional
work involved in supporting ARB_geometry_shader4.

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