Fixed several GES examples, but confused with GESAsset and GESClip

[Guofeng Zhang]

Hi,

test1.c worked well, it use:
    GESAsset *src_asset;
    ...
    src_asset = ges_asset_request (GES_TYPE_TEST_CLIP, NULL, NULL);
to instantiate a GESTestClip.

but test2.c and simple1.c does not work. They all use
    GESUriClip *src = ges_uri_clip_new (uri);

I tried to use
    GESAsset *src = ges_asset_request (GES_TYPE_URI_CLIP, uri, NULL);
but not succeeded.

The comment for ges_asset_request() describes that I should use 
ges_uri_clip_asset_request_sync, so I made the following change:
    GESAsset *src = ges_uri_clip_asset_request_sync( uri, &error) ;
    ...
    ges_layer_add_asset (layer, src, offset, 0, 5 * GST_SECOND, 
GES_TRACK_TYPE_AUDIO);
Now test2.c and simple1.c worked well.

What make me confused is
(1) ges_uri_clip_asset_request_sync() also use
    ges_asset_request (GES_TYPE_URI_CLIP, uri, &lerror)
to instantiate the GESAsset, but it works (mine not), why?

(2) The GESTAsset and GESClip has one to one relationship, that is, as 
documented that assets will represent for example a GESUriClips, 
GESBaseEffect etc.
each clip has a asset associated with it. But GESTestClip has no 
GESTestClipAsset (something like it) defined, how could an asset existed 
for GESTestClip? or testsrc does not need a resource to represent it, so 
it does not need a corresponding asset? if so, for appsrc, I could only 
define a clip for it and it will work with the GES framework?

Your help is appreciated very much.

Guofeng