Add ambient occlusion for blocks whose bottom faces completely cover the underlying block

[muzikbike]

The following blocks have bottom faces which 100% occlude the block below, yet of all of these, only the solid stone block causes any significant amount of ambient occlusion. The rest should probably be changed to do the same.

[MeeniMc]

Soul-soil blocks now cast ambient occlusion (as of 0.4.10, at least), which is identical to Vanilla. Rest of the proposed blocks still do not (just like vanilla).

[muzikbike]

With the fix to #904 eight-layer snow now casts a shadow as expected, but lower layers of snow still don't cast any shadow at all, which I think looks incorrect. I'd expect a shadow that increases in intensity for each layer, starting with barely visible for one layer up to the expected darkness for 8.

Here's some additional arrangements of snow layers - in each case, the center block is eight layers higher than the surrounding area. We'd therefore expect equal shadow sizes for each case, but that evidently isn't what happens.