basesrc: Handle tag and custom downstream events the same
basesrc: Handle tag and custom downstream events the same

Especially drop tag events when flushing to not send them over
and over again.

Should've been in the last commit already but I forgot to call
git rebase --continue...

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