buffer: improve flags
buffer: improve flags

Rename DROP to DECODE_ONLY.
Add DROPPABLE flag to mark buffers that can be dropped to save bandwidth without
destroying the stream.

lovebug356/gstreamer,lubosz/gstreamer,cfoch/gstreamer,collects/gstreamer,collects/gstreamer,StreamUtils/gstreamer,collects/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,lubosz/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,mparis/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,collects/gstreamer,ensonic/gstreamer,mparis/gstreamer,centricular/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,centricular/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,mparis/gstreamer,justinjoy/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,mparis/gstreamer,Lachann/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,centricular/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,krichter722/gstreamer,centricular/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,cfoch/gstreamer,justinjoy/gstreamer,krichter722/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,collects/gstreamer