Update few comments for 2x src/dst blending
Merge pull request #5740 from raven02/patch-35

Update few comments for 2x src/dst blending
Read Custom BGMs.
Ref JPCSP https://code.google.com/p/jpcsp/source/detail?r=3468
Fix issues in : https://github.com/hrydgard/ppsspp/issues/2929 and https://github.com/hrydgard/ppsspp/issues/5501
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Merge remote-tracking branch 'origin/master' into master_0.9.8