mesa: Track number of layers in layered framebuffers.

In order to properly clear layered framebuffers, we need to know how
many layers they have.  The easiest way to do this is to record it in
the gl_framebuffer struct when we check framebuffer completeness.

This patch replaces the gl_framebuffer::Layered boolean with a
gl_framebuffer::NumLayers integer, which is 0 if the framebuffer is
not layered, and equal to the number of layers otherwise.

v2: Remove gl_framebuffer::Layered and make gl_framebuffer::NumLayers
always have a defined value.  Fix factor of 6 error in the number of
layers in a cube map array.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
meta: fix meta clear of layered framebuffers

From section 4.4.7 (Layered Framebuffers) of the GLSL 3.2 spec:

    When the Clear or ClearBuffer* commands are used to clear a
    layered framebuffer attachment, all layers of the attachment are
    cleared.

This patch fixes meta clears to properly clear all layers of a layered
framebuffer attachment.  We accomplish this by adding a geometry
shader to the meta clear program which sets gl_Layer to a uniform
value.  When clearing a layered framebuffer, we execute in a loop,
setting the uniform to point to each layer in turn.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
i965: refactor blorp clear code in preparation for layered clears.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
i965: Fix blorp clear of layered framebuffers.

From section 4.4.7 (Layered Framebuffers) of the GLSL 3.2 spec:

    When the Clear or ClearBuffer* commands are used to clear a
    layered framebuffer attachment, all layers of the attachment are
    cleared.

This patch fixes the blorp clear path for color buffers.

Fixes piglit test "spec/!OpenGL 3.2/layered-rendering/clear-color".

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
i965: Fix fast clear of depth buffers.

From section 4.4.7 (Layered Framebuffers) of the GLSL 3.2 spec:

    When the Clear or ClearBuffer* commands are used to clear a
    layered framebuffer attachment, all layers of the attachment are
    cleared.

This patch fixes the fast depth clear path.

Fixes piglit test "spec/!OpenGL 3.2/layered-rendering/clear-depth".

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