send stream-start event to donstream when app tries to append data after EOS

[kkanag314]

Test Procedure:

• Launch the MSNow app using
• { "url":"https://spinco.staging.smarttv.viewlift.com/viewlift-bitmovin-player/sample-project/index.html" }
• After app is launched, click on 'MS NOW Linear"
• Wait about 5-10mins for advertisement to play
• Green screen is appearing when advertisement starts.

Root Cause:
During MediaSource reuse across ad/content transitions, the pipeline remained in EOS state after EOS was pushed. Since STREAM_START was not re-sent, subsequent CAPS and buffer pushes failed, causing the MediaSource to close and resulting in a green screen during stream transitions.

Fix Done:
Modified webKitMediaSrcLoop() to reset wasStreamStartSent on EOS and re-send STREAM_START when the MediaSource is reused. This allows the pipeline to recover from EOS state and correctly process new segments during period and ad transitions.

[eocanha]

I wonder if WebKit/WebKit@96f2c38, from upstream, might be a better solution for this problem. I'm going to explore backporting it to wpe-2.46 and check if that solves the problem.