i965: Remove *_generator::shader field; use prog field instead.

The "shader" field in fs_generator, vec4_generator, and gen8_generator
was only used for one purpose; to figure out if we were compiling an
assembly program or a GLSL shader (shader is NULL for assembly
programs).  And it wasn't being used properly: in vec4 shaders we were
always initializing it based on
prog->_LinkedShaders[MESA_SHADER_FRAGMENT], regardless of whether we
were compiling a geometry shader or a vertex shader.

This patch simplifies things by using the "prog" field instead; this
is also NULL for assembly programs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
mesa: Ensure that transform feedback refers to the correct program.

Previous to this patch, the _mesa_{Begin,Resume}TransformFeedback
functions were using ctx->Shader.CurrentProgram[MESA_SHADER_VERTEX] to
find the program that would be the source of transform feedback data.
This isn't correct--if there's a geometry shader present it should be
ctx->Shader.CurrentProgram[MESA_SHADER_GEOMETRY].  (These might be
different if separate shader objects are in use).

This patch creates a function get_xfb_source(), which figures out the
correct program to use based on GL state, and updates
_mesa_{Begin,Resume}TransformFeedback to call it.  get_xfb_source() is
written in terms of the gl_shader_stage enum, so it should not need
modification when we add tessellation shaders in the future.  It also
creates a new driver flag, NewTransformFeedbackProg, which is flagged
whenever this program changes.

To reduce future confusion, this patch also rewords some comments and
error message text to avoid referring to vertex shaders.

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

v2: make the for loop in get_xfb_source() clearer.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
i965/gen7: Use to the correct program when uploading transform feedback state.

Transform feedback may come from either the geometry shader or the
vertex shader, so we can't use
ctx->Shader.CurrentProgram[MESA_SHADER_VERTEX] to find the current
post-link transform feedback information.  Fortunately we can use
ctx->TransformFeedback.CurrentObject->shader_program.

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

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
!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.