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Add some benchmarker for the meshers

We want to make sure the meshing code is as fast as possible, but previously
it was basically impossible to determine how fast it was actually running,
or see if a change made a difference. This commit adds some simple
benchmarking code so we can tell when the meshing becomes faster or slower
after a change.

Right now it takes ~50ms to mesh a simple flat chunk on my computer. We can
probably get this down a fair bit.
Make more things use common.offsetIndex, removing lots of duplicated logic.
Move the block geometry creation logic to block.js

This will eventually allow us to reuse it for inventory.
Remove WTF. blockType CANNOT POSSIBLY be < 0 (it's a Uint8 array...).
Block models are sort of working, a bit hacky, not quite showing up as
expected. More testing needed.
Block items render w/ all faces now.

Man, I feel stupid. That took way longer than it should have to find...
Remove copied code, still need to work on organization.
Better block positioning.
Add TODO.
We only have one mesher now.
Kill the models we don't use anymore.
Merge pull request #161 from crazy2be/textured-inventory

Textured Inventory Blocks
Merge branch 'master' into terrain-blocks
Add the new blocks to the items.