intel: Return success when asked to allocate a 0-width/height renderbuffer.

It seems silly that GL lets you allocate these given that they're
framebuffer attachment incomplete, but the webgl conformance tests
actually go looking to see if the getters on 0-width/height
depth/stencil renderbuffers return good values.  By failing out here,
they all got smashed to 0, which turned out to be correct for all the
getters they tested except for GL_RENDERBUFFER_INTERNAL_FORMAT.  Now,
by succeeding but not making a miptree, that one also returns the
expected value.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit b7406404ab)

src/mesa/drivers/dri/intel/intel_fbo.c

@@ -237,6 +237,9 @@ intel_alloc_renderbuffer_storage(struct gl_context * ctx, struct gl_renderbuffer
        _mesa_lookup_enum_by_nr(internalFormat),
        _mesa_get_format_name(rb->Format), width, height);
 
+   if (width == 0 || height == 0)
+      return true;
+
    irb->mt = intel_miptree_create_for_renderbuffer(intel, rb->Format,
 						   width, height);
    if (!irb->mt)