Implement Format80 RLE compression

[pavlos256]

Makes the SHP writer a little smarter and the files produced a lot smaller.

This doesn't use all the encodings available but it's probably enough to close #5295.

[pchote]

This looks to write valid sprites, but I didn't compare file sizes.
Testcase:

• Open the asset browser, and observe that everything in mods/ra/bits looks fine.
• cd into the RA bits dir.
• Re-encode shp -> png -> shp using for i in $(ls *.shp); do mono ../../../OpenRA.Utility.exe --png $i cursor.pal; mono ../../../OpenRA.Utility.exe --shp "${i%%.*}-*"; done
• Check the asset browser again, and see that everything still looks fine.

[pchote]

You're only using blockLen once, so you can avoid the unnecessary scoping here and below by using offset - blockStart inline.

[pavlos256]

Updated with the requested changes.

For future reference, @Cr0s on IRC gave a link to a full Format80 encoder implementation but it's in Java. What I've implemented here is command 0xfe (0x80 we had already). A port should be pretty easy if anyone is interested.

[pchote]

https://github.com/OpenRA/OpenRA/wiki/Changelog/_compare/d83fd3b12d4ecc23d04bf04bccb035deeef5739e%5E...d83fd3b12d4ecc23d04bf04bccb035deeef5739e