Add PIGLIT_FORCE_WINDOW environment variable.

Setting PIGLIT_FORCE_WINDOW=1 causes all piglit windowsystem tests
(those that weren't invoked with the "-fbo" flag) to show the window
prior to calling piglit_display().  Setting PIGLIT_FORCE_WINDOW=0 (the
default) preserves piglit's existing behaviour, which is to only show
the window when the test is run in "manual" mode (with no "-auto"
argument supplied).

Setting PIGLIT_FORCE_WINDOW=1 slows down the piglit run, and produces
more annoying windows on the screen, but it has been observed to
produce more repeatable results when testing with the nVidia
proprietary driver for Linux.
Add PIGLIT_STRIP_ARG() macro for extracting args during piglit config.

v2: Fix update of *argc.
Use PIGLIT_STRIP_ARG() in formats.c
Support both core and compat contexts in xfb/overflow-edge-cases.c.

This will allow the test to be adapted for geometry shaders.

v2: Use VAO's if GL version >= 3.0.
gs: Adapt xfb/overflow-edge-cases.c for geometry shaders.

The transform feedback test "overflow-edge-cases" can now be invoked
with a "use_gs" argument, that causes it to use a geometry shader.
Support both core and compat contexts in xfb/intervening-read.c.

This will allow the test to be adapted for geometry shaders.

v2: Use VAO's if GL version >= 3.0.
gs: Adapt xfb/intervening-read.c for geometry shaders.

The transform feedback test "intervening-read" can now be invoked with
a "use_gs" argument, that causes it to use a geometry shader.

v2: In the geometry shader, make positions array const, and fix outer
loop bound.
gs: Make texelFetch and textureSize tests use GLSL 1.50 for geometry shaders.

Previously, these tests used ARB_geometry_shader4.  However, the
initial implementation of geometry shaders in Mesa isn't going to
support ARB_geometry_shader4.  So test this functionality using GLSL
1.50 geometry shaders instead.

v2: Use a generic vertex attribute instead of gl_PositionIn.
Basic test that transform feedback works with geometry shaders.
gs: Test rules related to geometry shader input sizes.

Section 4.3.8.1 (Input Layout Qualifiers) of the GLSL 1.50 spec
contains some tricky rules for how the sizes of geometry shader input
arrays are related to the input layout specification.  In essence,
those rules boil down to the following:

- If an input array declaration does not specify a size, and it
  follows an input layout declaration, it is sized according to the
  input layout.

- If an input layout declaration follows an input array declaration
  that didn't specify a size, the input array declaration is given a
  size at the time the input layout declaration appears.

- All input layout declarations and input array sizes must ultimately
  match.  Inconsistencies are reported as soon as they are detected,
  at compile time if the inconsistency is within one compilation unit,
  otherwise at link time.

- At least one compilation unit must contain an input layout
  declaration.

This patch includes tests for verifying all of these behaviours,
modelled as closely as possible from the text and examples in the GLSL
1.50 spec.  Both interface block style and and non-interface block
style declarations are tested.

v2: Test layout vs explicit size more thoroughly: don't just test the
case where one compilation unit specifies a layout with N vertices and
another compilation unit specifies an input with size >N.  Also test
the case where the second compilation unit specifies an input with
size <N.
Geometry shaders: add a basic EndPrimitive() test.
!UPSTREAM: GL 3.1: test that core features are properly excluded from old GLSL versions.

TODO:
- Do I need a better commit message?
- Test on nVidia
!UPSTREAM: gs: Test all core inputs except gl_PrimitiveIDIn.

gl_PrimitiveIDIn will need more aggressive testing, so that will be
addressed in a separate patch.

TODO:
- Add core outputs (gl_ClipDistance, gl_PointSize, and gl_PrimitiveID already done)
gs: Test gl_PrimitiveIDIn
!UPSTREAM: Test core GS inputs on GLSL 1.50