Rock floor on cave levels

[BenCheung0422]

Is your feature request related to a problem? Please describe.
Cave levels are filled with dirt tiles, but this may allow plants growing in these levels. We have rock tiles, but they are placed on dirt tiles. Therefore, the levels are composited by dirt-rock-dirt-rock-dirt instead of rock-rock-rock-rock-dirt, which may not match the geology in sense. Some features may be related to #396.

Describe the solution you'd like
Cave levels should be filled with rock floors instead of dirt tiles. Rock tiles can be separated to floor form or wall form, so that these can exist as floors or walls (the current rock tiles).