i965/gen7: Emit workaround flush when changing GS enable state.

v2: Don't go to extra work to avoid extraneous flushes.  (Previous
experiments in the kernel have suggested that flushing the pipeline
when it is already empty is extremely cheap).

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

Reviewed-by: Eric Anholt <eric@anholt.net>
!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.
!UPSTREAM: Prohibit illegal mixing of redeclarations inside/outside gl_PerVertex.

From section 7.1 (Built-In Language Variables) of the GLSL 4.10
spec:

    Also, if a built-in interface block is redeclared, no member of
    the built-in declaration can be redeclared outside the block
    redeclaration.

We have been regarding this text as a clarification to the behaviour
established for gl_PerVertex by GLSL 1.50, so we apply it regardless
of GLSL version.

This patch enforces a stricter rule: no built-in shader input or
output may be redeclared more than once.

It does so by adding a boolean to ir_variable to track whether the
variable is an implicitly-defined builtin.  For built-ins, this
boolean starts out as true.  When a built-in is redeclared, we set it
to false.  If the built-in is redeclared again, we notice that the
boolean is false and generate a compile error.

Fixes piglit tests:
- gs-redeclares-pervertex-out-after-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-global-redeclaration.vert
- gs-redeclares-pervertex-out-after-other-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-other-global-redeclaration.vert
- gs-redeclares-pervertex-out-before-global-redeclaration
- vs-redeclares-pervertex-out-before-global-redeclaration

Regresses piglit tests:
- glslparsertest/CorrectUnsizedArray.frag
- spec/glsl-1.50/compiler/layout-any-order-ids.frag

For reference, these tests pass using the Nvidia proprietary driver
for Linux (version 313.18):
- gs-redeclares-pervertex-out-before-global-redeclaration
- vs-redeclares-pervertex-out-before-global-redeclaration
- glslparsertest/CorrectUnsizedArray.frag

And these tests fail:
- gs-redeclares-pervertex-out-after-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-global-redeclaration.vert
- gs-redeclares-pervertex-out-after-other-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-other-global-redeclaration.vert
- spec/glsl-1.50/compiler/layout-any-order-ids.frag
Revert "!UPSTREAM: Prohibit illegal mixing of redeclarations inside/outside gl_PerVertex."

This reverts commit b09c94845d53fb0106f3120f5e77fbdafd878be5.