mesa: Set transform feedback's default buffer mode to INTERLEAVED_ATTRIBS

Fixes part of es3conform's transform_feedback_init_defaults test.
NOTE: This is a candidate for the stable branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit 13f9012ad3)

src/mesa/main/shaderobj.c

@@ -250,6 +250,8 @@ _mesa_init_shader_program(struct gl_context *ctx, struct gl_shader_program *prog
    prog->Geom.OutputType = GL_TRIANGLE_STRIP;
 #endif
 
+   prog->TransformFeedback.BufferMode = GL_INTERLEAVED_ATTRIBS;
+
    prog->InfoLog = ralloc_strdup(prog, "");
 }