mesa: Set the correct initial value of the texture buffer object format.

Fixes piglit GL_ARB_texture_buffer_object/get

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/texobj.c

@@ -153,6 +153,7 @@ _mesa_initialize_texture_object( struct gl_texture_object *obj,
    obj->Swizzle[3] = GL_ALPHA;
    obj->_Swizzle = SWIZZLE_NOOP;
    obj->Sampler.sRGBDecode = GL_DECODE_EXT;
+   obj->BufferObjectFormat = GL_LUMINANCE8;
 }