mesa, glsl_to_tgsi: Add new gl_context::NativeIntegers flag.

mesa, glsl_to_tgsi: Add new gl_context::NativeIntegers flag.

Previously, native integer support was based on whether the driver
advertised GLSL 1.30 or not.  However, drivers that natively support
integers may wish to do so for older GLSL versions as well.  Adding this
new opt-in flag allows them to do so.

Currently disabled by default on all drivers, which was the existing
behavior (no drivers currently implement GLSL 1.30).

Fixes piglit tests on i965 with INTEL_GLSL_VERSION=130 set:
- spec/glsl-1.10/fs-uniform-int-110.shader_test
- spec/glsl-1.30/fs-uniform-int-130.shader_test
(it was doubly converting the data)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>