mesa: Ensure that transform feedback refers to the correct program.

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>