mesa: refactor _mesa_compute_max_transform_feedback_vertices from i965.

mesa: refactor _mesa_compute_max_transform_feedback_vertices from i965.

Previously, the i965 driver contained code to compute the maximum
number of vertices that could be written without overflowing any
transform feedback buffers.  This code wasn't driver-specific, and for
GLES3 support we're going to need to use it in core mesa.  So this
patch moves the code into a core mesa function,
_mesa_compute_max_transform_feedback_vertices().

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
v2: Eliminate C++-style variable declarations, since these won't work
with MSVC.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>