glsl: Don't generate misleading debug names when packing gs inputs.

Previously, when packing geometry shader input varyings like this:

    in float foo[3];
    in float bar[3];

lower_packed_varyings would declare a packed varying like this:

    (declare (shader_in flat) (array ivec4 3) packed:foo[0],bar[0])

That's confusing, since the packed varying acutally stores all three
values of foo and all three values of bar.

This patch causes it to generate the more sensible declaration:

    (declare (shader_in flat) (array ivec4 3) packed:foo,bar)

Note that there should be no functional change for users of geometry
shaders, since the packed name is only used for generating debug
output.  But this should reduce confusion when using INTEL_DEBUG=gs.
glsl: Rework interface block linking.

Previously, when doing intrastage and interstage interface block
linking, we only checked the interface type; this prevented us from
catching some link errors.

We now check the following additional constraints:

- For intrastage linking, the presence/absence of interface names must
  match.

- For shader ins/outs, the interface names themselves must match when
  doing intrastage linking (note: it's not clear from the spec whether
  this is necessary, but Mesa's implementation currently relies on
  it).

- Array vs. nonarray must be consistent, taking into account the
  special rules for vertex-geometry linkage.

- Array sizes must be consistent (exception: during intrastage
  linking, an unsized array matches a sized array).

Note: validate_interstage_interface_blocks currently handles both
uniforms and in/out variables.  As a result, if all three shader types
are present (VS, GS, and FS), and a uniform interface block is
mentioned in the VS and FS but not the GS, it won't be validated.  I
plan to address this in later patches.

Fixes the following piglit tests in spec/glsl-1.50/linker:
- interface-blocks-vs-fs-array-size-mismatch
- interface-vs-array-to-fs-unnamed
- interface-vs-unnamed-to-fs-array
- intrastage-interface-unnamed-array
!UPSTREAM: i965/gs: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
!UPSTREAM: glsl/linker: Fail if linking geometry shader with 0 GEOMETRY_VERTICES_OUT.

ARB_geometry_shader4 spec 2.15.2:
"Linking will also fail [...] if the program object contains objects to form a
geometry shader, and the value of GEOMETRY_VERTICES_OUT_ARB is zero."

Reviewed-by: Paul Berry <stereotype441@gmail.com>

TODO: May not be necessary for GLSL 1.50 style geometry shaders.  Make a test to validate.
!UPSTREAM: Debug optimization passes.
!UPSTREAM: Work around bug in AntTweakBar

AntTweakBar tries to use glGetString(GL_EXTENSIONS) when connected to
a core context (it should use glGetStringi()).  Go ahead and allow
this for the time being.