Rework TMX struct

[Daft-Freak]

This is the more-breaky (but also fixes more) alternative to #76
Requires 32blit/[email protected]:tiled-new-struct

From the comment there:

* Fixes `empty_tile` for 16 bit

* Adds `flags` so that 16 bit/transforms can be detected

* Only include the transforms with the struct if any are non-zero (always include for the "raw" output)

* More validation for `load_tmx`

* Adds a length field so that any future changes can be less breaky

* Happens to make the struct a nice round 16 bytes... (and fixes the alignment)

* Breaks the TMX struct

* Guess it should be using construct...

[coveralls]

Pull Request Test Coverage Report for Build 741392978

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 72.793%

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Files with Coverage Reduction	New Missed Lines	%
ttblit/asset/builders/map.py	1	97.92%

Totals
Change from base Build 664499501:	0.1%
Covered Lines:	907
Relevant Lines:	1246

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💛 - Coveralls

[Gadgetoid]

Time to explode Super Square Bros. Thank you 😆

Heads up @ThePythonator