glsl: Fix interstage uniform interface block link error detection.

Previously, we checked for interstage uniform interface block link
errors in validate_interstage_interface_blocks(), which is only called
on pairs of adjacent shader stages.  Therefore, we failed to detect
uniform interface block mismatches between non-adjacent shader stages.

Before the introduction of geometry shaders, this wasn't a problem,
because the only supported shader stages were vertex and fragment
shaders, therefore they were always adjacent.  However, now that we
allow a program to contain vertex, geometry, and fragment shaders,
that is no longer the case.

Fixes piglit test "skip-stage-uniform-block-array-size-mismatch".

Cc: "10.0" <mesa-stable@lists.freedesktop.org>

v2: Rename validate_interstage_interface_blocks() to
validate_interstage_inout_blocks() to reflect the fact that it no
longer validates uniform blocks.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
!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: 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.
!UPSTREAM: main: check texture target when validating layered framebuffers.

From section 4.4.4 (Framebuffer Completeness) of the GL 3.2 spec:

    If any framebuffer attachment is layered, all populated
    attachments must be layered. Additionally, all populated color
    attachments must be from textures of the same target.

We weren't checking that the attachments were from textures of the
same target.

TODO:
- Make a piglit test for this, and validate it on Nvidia.