gstreamer的video/x-raw-rgb插件

"video/x-raw-rgb"是GstCapsFilter插件， 这是个格式过滤插件，其实就是对pipeline中的数据流的格式做一个限制。

 plugin coreelements的features（17 elements）如下：

  valve: Valve element

  multiqueue: MultiQueue

  typefind: TypeFind

  tee: Tee pipe fitting

  filesink: File Sink

  queue2: Queue 2

  queue: Queue

  output-selector: Output selector

  input-selector: Input selector

  identity: Identity

  funnel: Funnel pipe fitting

  filesrc: File Source

  fdsink: Filedescriptor Sink

  fdsrc: Filedescriptor Source

  fakesink: Fake Sink

  fakesrc: Fake Source

  capsfilter: CapsFilter