Lower-left transparency fixes

While working with sprite_add, I noticed a small typo that makes it so certain colors in the lower-left will not make the background transparent.

I also decided to check on background_add to see if it had the same problem and discovered that it doesn't even handle lower-left transparency at all.  I went ahead and implemented it following the same code as sprite_add (including the fix.)

Additionally, since I couldn't submit a pull request without also committing "Treat <> as != rather than error", the changes included allow games written with "<>" comparisons in the code to interpret it as a "!=" rather than "< >", which causes an error.  I'm not sure if there is a specific reason this wasn't ever handled, but 90% of my games written 8+ years ago use "<>" instead of "!=", so I decided to make this change as a matter of convenience.

All of the submitted changes work fine on Windows 7 and have been tested with several games without issue, but I don't have a way to test other platforms at the moment.  The code should be fairly explanatory though.