glsl: Remove unused field loop_variable_state::loop.

This field was neither initialized nor used.  It was just dead memory.
i965/gen6: Fix multisample resolve blits for luminance/intensity 32F formats.

On gen6, multisamble resolve blits use the SAMPLE message to blend
together the 4 samples for each texel.  For some reason, SAMPLE
doesn't blend together the proper samples when the source format is
L32_FLOAT or I32_FLOAT, resulting in blocky artifacts.

To work around this problem, sample from the source surface using
R32_FLOAT.  This shouldn't affect rendering correctness, because when
doing these resolve blits, the destination format is R32_FLOAT, so the
channel replication done by L32_FLOAT and I32_FLOAT is unnecessary.

Fixes piglit tests on Sandy Bridge:
- spec/ARB_texture_float/multisample-formats 2 GL_ARB_texture_float
- spec/ARB_texture_float/multisample-formats 4 GL_ARB_texture_float

No piglit regressions on Sandy Bridge.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70601

Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: 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.
!UPSTREAM: WIP fix for 72026
!UPSTREAM: glsl: Make hierarchical visitor visit ir_loop::counter.

Fixes reparenting of ir_loops whose counter is non-NULL.
!UPSTREAM: Teach ir_variable_refcount about implicit accesses to ir_loop::counter variables.
!UPSTREAM: print blorp blit surface formats
!UPSTREAM: In ir_validate, check that ir_loop::counter always refers to a new var.