Fix flickering on the loading screen.
Fix flickering on the loading screen.

The buffer was not being cleared at the start of the frame,
so it was being filled with garbage. This was particularly
noticable on Macs.

Change-Id: If318999b6f9760a27db8b1b709cdbd8563ca838b
Tested: No longer flickers on Mac.

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi