vaapipostproc: can't negotiate dmabuf in sink pad

consider this pipeline

gst-launch-1.0 gltestsrc ! video/x-raw\(memory:GLMemory\),width=512,height=256,framerate=50/1,format=RGBA ! gldownload ! vaapipostproc ! fakesink -v

It doesn't work since vaapipostproc can't negotiate with the offered caps by gldownload, since there is no intersection:

video/x-raw(memory:GLMemory), width=(int)512, height=(int)256, framerate=(fraction)50/1, format=(string)RGBA, texture-target=(string)2D; video/x-raw(memory:DMABuf), width=(int)512, height=(int)256, framerate=(fraction)50/1, format=(string)RGBA; video/x-raw, width=(int)512, height=(int)256, framerate=(fraction)50/1, format=(string)RGBA

There are no intersection since vaapipostproc only propose memory:VASurface and raw caps-features. There are missing bits for adding memory:DMABuffer in the caps transformation.