d3d11videosink: Add support for drawing on application's own texture

    examples: Add d3d11videosink examples for shared-texture use cases

    Add two examples to demonstrate "draw-on-shared-texture" use cases.

    d3d11videosink will draw application's own texture without copy
    by using:
    - Enable "draw-on-shared-texture" property
    - make use of "begin-draw" and "draw" signals

    And then, application will render the shared application's texture
    to swapchain's backbuffer by using
    1) Direct3D11 APIs
    2) Or, Direct3D9Ex + interop APIs

    d3d11videosink: Add support for drawing on application's own texture

    Add a way to support drawing on application's texture instead of
    usual window handle.
    To make use of this new feature, application should follow below step.
    1) Enable this feature by using "draw-on-shared-texture" property
    2) Watch "begin-draw" signal
    3) On "begin-draw" signal handler, application can request drawing
       by using "draw" signal action. Note that "draw" signal action
       should be happen before "begin-draw" signal handler is returned

    NOTE 1) For texture sharing, creating a texture with
    D3D11_RESOURCE_MISC_SHARED_KEYEDMUTEX flag is strongly recommend
    if possible because we cannot ensure sync a texture
    which was created with D3D11_RESOURCE_MISC_SHARED
    and it would cause glitch with ID3D11VideoProcessor use case.

    NOTE 2) Direct9Ex doesn't support texture sharing which was
    created with D3D11_RESOURCE_MISC_SHARED_KEYEDMUTEX. In other words,
    D3D11_RESOURCE_MISC_SHARED is the only option for Direct3D11/Direct9Ex interop.

    NOTE 3) Because of missing synchronization around ID3D11VideoProcessor,
    If shared texture was created with D3D11_RESOURCE_MISC_SHARED,
    d3d11videosink might use fallback texture to convert DXVA texture
    to normal Direct3D texture. Then converted texture will be
    copied to user-provided shared texture.

    * Why not use generic appsink approach?
    In order for application to be able to store video data
    which was produced by GStreamer in application's own texture,
    there would be two possible approaches,
    one is copying our texture into application's own texture,
    and the other is drawing on application's own texture directly.
    The former (appsink way) cannot be a zero-copy by nature.
    In order to support zero-copy processing, we need to draw on
    application's own texture directly.

    For example, assume that application wants RGBA texture.
    Then we can imagine following case.

    "d3d11h264dec ! d3d11convert ! video/x-raw(memory:D3D11Memory),format=RGBA ! appsink"
                                 ^
                                 |_ allocate new Direct3D texture for RGBA format

    In above case, d3d11convert will allocate new texture(s) for RGBA format
    and then application will copy again the our RGBA texutre into
    application's own texture. One texture allocation plus per frame GPU copy will hanppen
    in that case therefore.
    Moreover, in order for application to be able to access
    our texture, we need to allocate texture with additional flags for
    application's Direct3D11 device to be able to read texture data.
    That would be another implementation burden on our side

    But with this MR, we can configure pipeline in this way
    "d3d11h264dec ! d3d11videosink".

    In that way, we can save at least one texture allocation and
    per frame texutre copy since d3d11videosink will convert incoming texture
    into application's texture format directly without copy.

    * What if we expose texture without conversion and application does
      conversion by itself?
    As mentioned above, for application to be able to access our texture
    from application's Direct3D11 device, we need to allocate texture
    in a special form. But in some case, that might not be possible.
    Also, if a texture belongs to decoder DPB, exposing such texture
    to application is unsafe and usual Direct3D11 shader cannot handle
    such texture. To convert format, ID3D11VideoProcessor API needs to
    be used but that would be a implementation burden for application.

Closes: #1407 (closed)