libvpx

Add a guard on intra mode skip control for RTC mode

This commit adds a guard condition to the intra mode test skip
control in RTC coding mode. If all inter modes are skipped, force
the encoder to check intra mode. It avoids situations where the
encoder processes without properly assigning required mode
information.

Change-Id: Ibb349fee997d6584ce901d08b06e8df3ca9c01b1