pad: not only describe conditions in the docs, also check them in the code
pad: not only describe conditions in the docs, also check them in the code

When blocking pads, check if the pad is in the rigt direction. Log some info
for the developer and return FALSE, instead of just locking up.

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