libs: encoder: vp8,vp9: reset frame_counter when input frame's format changes

When input frame's formate changes, vp{8,9} encoders don't reset their frame
counter, hence the newly created frame could become a P-frame, leading to some
major troubles (sigabrt in libdrm in case of vp9). This patch adds some frame
prediction-related reset logic to the `flush' methods of GstVaapiEncoderVP8 and
GstVaapiEncoderVP9 implementations.
8 jobs for encoder_vp8_vp9_dynamic_res in 21 minutes and 59 seconds (queued for 1 minute and 31 seconds)
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