i965 gen6: Initial implementation of transform feedback.

This patch adds basic transform feedback capability for Gen6 hardware.
This consists of several related pieces of functionality:

(1) In gen6_sol.c, we set up binding table entries for use by
transform feedback.  We use one binding table entry per transform
feedback varying (this allows us to avoid doing pointer arithmetic in
the shader, since we can set up the binding table entries with the
appropriate offsets and surface pitches to place each varying at the
correct address).

(2) In brw_context.c, we advertise the hardware capabilities, which
are as follows:

   MAX_TRANSFORM_FEEDBACK_INTERLEAVED_COMPONENTS 64
   MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS        4
   MAX_TRANSFORM_FEEDBACK_SEPARATE_COMPONENTS    16

OpenGL 3.0 requires these values to be at least 64, 4, and 4,
respectively.  The reason we advertise a larger value than required
for MAX_TRANSFORM_FEEDBACK_SEPARATE_COMPONENTS is that we have already
set aside 64 binding table entries, so we might as well make them all
available in both separate attribs and interleaved modes.

(3) We set aside a single SVBI ("streamed vertex buffer index") for
use by transform feedback.  The hardware supports four independent
SVBI's, but we only need one, since vertices are added to all
transform feedback buffers at the same rate.  Note: at the moment this
index is reset to 0 only when the driver is initialized.  It needs to
be reset to 0 whenever BeginTransformFeedback() is called, and
otherwise preserved.

(4) In brw_gs_emit.c and brw_gs.c, we modify the geometry shader
program to output transform feedback data as a side effect.

(5) In gen6_gs_state.c, we configure the geometry shader stage to
handle the SVBI pointer correctly.

Note: ordering of vertices is not yet correct for triangle strips
(alternate triangles are improperly oriented).  This will be addressed
in a future patch.

Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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