[Ohrrpgce] SVN: teeemcee/11144 Fix battle backdrops being mispositioned when game resolution isn't 320x

[subversion at HamsterRepublic.com]

teeemcee
2019-05-01 04:02:47 -0700 (Wed, 01 May 2019)
354
Fix battle backdrops being mispositioned when game resolution isn't 320x200

Battle backdrops are centered (unlike out-of-battle backdrops which are aligned
to the top-left, should we change battle backdrops to match?), but the battle
root slice size didn't match the size of the video page (320x200 or smaller),
it was equal in size to the whole window.
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U   wip/bmod.rbas
U   wip/common.rbas
U   wip/slices.bas
U   wip/slices.bi