glsl: Teach ir_variable_refcount about ir_loop::counter variables.

If an ir_loop has a non-null "counter" field, the variable referred to
by this field is implicitly read and written by the loop.  We need to
account for this in ir_variable_refcount, otherwise there is a danger
we will try to dead-code-eliminate the loop counter variable.

Note: at the moment the dead code elimination bug doesn't occur due to
a bug in ir_hierarchical_visitor: it doesn't visit the "counter"
field, so dead code elimination doesn't treat it as a candidate for
elimination.  But the patch to follow will fix that bug, so we need to
fix ir_variable_refcount first in order to avoid breaking dead code
elimination.
glsl: Fix inconsistent assumptions about ir_loop::counter.

The compiler back-ends (i965's fs_visitor and brw_visitor,
ir_to_mesa_visitor, and glsl_to_tgsi_visitor) assume that when
ir_loop::counter is non-null, it points to a fresh ir_variable that
should be used as the loop counter (as opposed to an ir_variable that
exists elsewhere in the instruction stream).

However, previous to this patch:

(1) loop_control_visitor did not create a new variable for
    ir_loop::counter; instead it re-used the existing ir_variable.
    This caused the loop counter to be double-incremented (once
    explicitly by the body of the loop, and once implicitly by
    ir_loop::increment).

(2) ir_clone did not clone ir_loop::counter properly, resulting in the
    cloned ir_loop pointing to the source ir_loop's counter.

(3) ir_hierarchical_visitor did not visit ir_loop::counter, resulting
    in the ir_variable being missed by reparenting.

Additionally, most optimization passes (e.g. loop unrolling) assume
that the variable mentioned by ir_loop::counter is not accessed in the
body of the loop (an assumption which (1) violates).

The combination of these factors caused a perfect storm in which the
code worked properly nearly all of the time: for loops that got
unrolled, (1) would introduce a double-increment, but loop unrolling
would fail to notice it (since it assumes that ir_loop::counter is not
accessed in the body of the loop), so it would unroll the loop the
correct number of times.  For loops that didn't get unrolled, (1)
would introduce a double-increment, but then later when the IR was
cloned for linking, (2) would prevent the loop counter from being
cloned properly, so it would look to further analysis stages like an
independent variable (and hence the double-increment would stop
occurring).  At the end of linking, (3) would prevent the loop counter
from being reparented, so it would still belong to the shader object
rather than the linked program object.  Provided that the client
program didn't delete the shader object, the memory would never get
reclaimed, and so the shader would function properly.

However, for loops that didn't get unrolled, if the client program did
delete the shader object, and the memory belonging to the loop counter
got re-used, this could cause a use-after-free bug, leading to a
crash.

This patch fixes loop_control_visitor, ir_clone, and
ir_hierarchical_visitor to treat ir_loop::counter the same way the
back-ends treat it: as a freshly allocated ir_variable that needs to
be visited and cloned independently of other ir_variables.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72026
glsl: In ir_validate, check that ir_loop::counter always refers to a new var.

The compiler back-ends (i965's fs_visitor and brw_visitor,
ir_to_mesa_visitor, and glsl_to_tgsi_visitor) have been assuming this
for some time.  Thanks to the preceding patch, the compiler front-end
no longer breaks this assumption.

This patch adds code to validate the assumption so that if we have
future bugs, we'll be able to catch them earlier.
glsl: Improve documentation of ir_loop counter/control fields.
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: print blorp blit surface formats