[Libva] VA/EGL backend

[Xiang, Haihao]

Hi, 
   I created a new branch vaapi-egl which introduces some new hooks for
VA/EGL backend.  vaapi-egl has a generic implementation with native
pixamp. The backend-driver (intel-drv/pvr-drv) can provide its specific
implementation.  There is a test case va_egl_x11.c under test/egl to
show how to use vaapi-egl. 

Thanks
Haihao

/**
 * Return a suitable VADisplay for VA API
 *
 * @param[in] native_dpy the native display
 * @param[in] egl_dpy the EGL display
 * @return a VADisplay
 */
VADisplay vaGetDisplayEGL(
    VANativeDisplay native_dpy,
    EGLDisplay egl_dpy
);

/**
 * Create a surface used for display to OpenGL ES
 *
 * The application shall maintain the live EGL context itself.
 *
 * @param[in]  dpy        the VA display
 * @param[in]  target     the GL target to which the texture needs to be bound, must be GL_TEXTURE_2D
 * @param[in]  texture    the GL texture
 * @param[in]  width      the surface width
 * @param[in]  height     the surface height
 * @param[out] gl_surface the VA/EGL surface
 * @return VA_STATUS_SUCCESS if successful
 */
VAStatus vaCreateSurfaceEGL(
    VADisplay dpy,
    GLenum target,
    GLuint texture,
    unsigned int width,
    unsigned int height,
    VASurfaceEGL *gl_surface
);

/**
 * Destroy a VA/EGL surface
 *
 * The application shall maintain the live EGL context itself.
 *
 * @param[in]  dpy        the VA display
 * @param[in]  gl_surface the VA surface
 * @return VA_STATUS_SUCCESS if successful
 */
VAStatus vaDestroySurfaceEGL(
    VADisplay dpy,
    VASurfaceEGL gl_surface
);

/**
 * Associate a EGL surface with a VA surface
 *
 * @param[in]  dpy         the VA display
 * @param[in]  egl_surface the VA/EGL destination surface
 * @param[in]  surface     the VA surface
 * @param[in]  flags       the flags to PutSurface
 * @return VA_STATUS_SUCCESS if successful
 */
VAStatus vaAssociateSurfaceEGL(
    VADisplay dpy,
    VASurfaceEGL egl_surface,
    VASurfaceID surface,
    unsigned int flags
);

/**
 * Update the content of a VA/EGL surface
 *
 * Changes to VA surface are committed to VA/EGL surface at this point.
 *
 * @param[in]  dpy         the VA display
 * @param[in]  egl_surface the VA/EGL destination surface
 * @return VA_STATUS_SUCCESS if successful
 */
VAStatus vaUpdateAssociatedSurfaceEGL(
    VADisplay dpy,
    VASurfaceEGL egl_surface
);

/**
 * Deassociate a EGL surface
 *
 * @param[in]  dpy         the VA display
 * @param[in]  egl_surface the VA/EGL destination surface
 * @return VA_STATUS_SUCCESS if successful
 */
VAStatus vaDeassociateSurfaceEGL(
    VADisplay dpy,
    VASurfaceEGL egl_surface
);