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.
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.
glsl: Simplify aggregate type inference to prepare for ARB_arrays_of_arrays.

Most of the time it is not necessary to perform type inference to
compile GLSL; the type of every expression can be inferred from the
contents of the expression itself (and previous type declarations).
The exception is aggregate initializers: their type is determined by
the LHS of the variable being assigned to.  For example, in the
statement:

   mat2 foo = { { 1, 2 }, { 3, 4 } };

the type of { 1, 2 } is only known to be vec2 (as opposed to, say,
ivec2, uvec2, int[2], or a struct) because of the fact that the result
is being assigned to a mat2.

Previous to this patch, we handled this situation by doing some type
inference during parsing: when parsing a declaration like the one
above, we would call _mesa_set_aggregate_type(), which would infer the
type of each aggregate initializer and store it in the corresponding
ast_aggregate_initializer::constructor_type field.  Since this
happened at parse time, we couldn't do the type inference using
glsl_type objects; we had to use ast_type_specifiers, which are much
more awkward to work with.  Things are about to get more complicated
when we add support for ARB_arrays_of_arrays.

This patch simplifies things by postponing the call to
_mesa_set_aggregate_type() until ast-to-hir time, when we have access
to glsl_type objects.  As a side benefit, we only need to have one
call to _mesa_set_aggregate_type() now, instead of six.
!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.