Fix incorrect overlay pixel format selection, buffer mapping
Jeffrey Lee authored
Detail:
  s/GVOverlay:
  - Fix register mixup in GV_MapOverlayBuffer which would cause too little memory to be mapped in for YUV overlays.
  - Fix OverlayCreateVetCommon using a wrong default mode flags value, causing 32K LTBGR to be erroneously detected as a valid format if the mode selector block didn't explicitly specify the mode flags.
  - Add explicit failure check to CreateOverlayBuffer (for e.g. out of memory), previously failure was only being detected as a side-effect of the mailbox GetDispmanxMemhandle message failing
Admin:
  Tested on Rasbperry Pi 3
  Fixes a couple of issues raised in this thread:
  https://www.riscosopen.org/forum/forums/3/topics/11504


Version 0.53. Tagged as 'BCMVideo-0_53'
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