nv50: add support for texelFetch'ing MS textures, ARB_texture_multisample

Creates two areas in the AUX constbuf:
 - Sample offsets for MS textures
 - Per-texture MS settings

When executing a texelFetch with a MS sampler, looks up that texture's
settings and adjusts the parameters given to the texfetch instruction.

With this change, all the ARB_texture_multisample piglits pass, so turn
on PIPE_CAP_TEXTURE_MULTISAMPLE.

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

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
nv50: enable seamless cube maps on all hw

Some of the hardware support is missing. The NVIDIA-provided driver,
which claims seamless cube map support fails the relevant tests as well.
As this is the last extension before we can have OpenGL 3.2, doing 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>