Add support of old MCRegion (.mcr) format

[Low-power]

This old region/chunk format is used by Minecraft Beta 1.3 and up to Minecraft 1.2; it is supported by early versions of TMCMR until commit 2294e4b.

This work has ported 2 classes back from commit 2014457 to support the old format. When searching regions in a directory, the old MCRegion format files will not be used if any Anvil format files are found.

I has used my forked version to render my Beta 1.6 world map, which is available at https://beijing.rivoreo.one:2037/b/