Define what to make of all the existing levels

[akien-mga]

Partly related to #10. There are many levels, some of which are probably what was considered "good" before development stopped, and others were probably considered less interesting.

Maybe @reduz or @punto- can already shed some light on this, but otherwise/additionally, we should test them all to make our own opinion and see the patterns used in them.

Then we should decide what to make of each them:

• Improve the level to make it one of the "final" levels for the first release
• Keep it around as a showcase of a gameplay mechanism or as test level
• Drop it if it's actually not funny and was an attempt at using a gameplay mechanism that doesn't work so well

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Also several tilemaps have issues with flipped tiles that are bigger than a 1x1 tile, due to a fix in 2.0 that this specific use case in older games. I fixed quite a few of them manually, but not all levels; we should probably assess these levels before trying to fix them all as it's really time consuming.