glsl: Initialize samplers to 0, propagate sampler values to the gl_program

glsl: Initialize samplers to 0, propagate sampler values to the gl_program

The spec requires that samplers be initialized to 0.  Since this
differs from the 1-to-1 mapping of samplers to texture units assumed
by ARB assembly shaders (and the gl_program structure), be sure to
propagate this date from the gl_shader_program to the gl_program.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
CC: Vadim Girlin <vadimgirlin@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49088