mesa: Use correct data for MAX_{VERTEX,GEOMETRY}_VARYING_COMPONENTS_ARB queries

mesa: Use correct data for MAX_{VERTEX,GEOMETRY}_VARYING_COMPONENTS_ARB queries

Previously gl_constants::MaxVaryingComponents was used.  Now
gl_constants::VertexProgram::MaxOutputs and
gl_constants::GeometryProgram::MaxOutputs are used.

This means that st_extensions.c had to be updated to set these fields
instead of MaxVaryingComponents.  It was previously the only place that
set MaxVaryingComponents.

I believe that the structure is allocated by calloc, so the value should
be initialized to zero in non-Gallium drivers before and after my
change.  Right now nobody enables GL_ARB_geometry_shader4, so it's
pretty much dead code anyway.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Cc: Zack Rusin <zackr@vmware.com>