Replaced Block enum with BlockType and Block classes, allowing blocks
to be defined at run time.

BlockType holds all constant state which is shared among all blocks of
the same type. For now, this is indices into the texture array. Block
holds all per-block information. At the moment, this is only a pointer to
BlockType, but this could eventually also hold things like orientation
or a pointer to some block-specific larger structure.
Small cleanups to Chunk.
Use dirt texture on dirt blocks and the underside of grass blocks.
Hack out simple constant frame rate loop with ASIO.
Replaced Block::Pos with glm::ivec3. Moved Face to util.h, and moved
functions involving Face and Block::Pos to util.{h,cpp}.

Several of the functions that involved Face are applicable beyond
positions in chunks. In particular, they'll appear in code that deals
with chunks' positions in the ChunkGrid. Once these were seperate from
Chunk::Pos, Chunk::Pos was a glorified glm::ivec3, so eliminating it made
these functions usable in the same fashion while still being general enough
to apply to the future ChunkGrid.
GLEW doesn't support core profiles.

This was causing errors on my new Nvidia desktop, because it actually
enforces a core profile and prevents GLEW from working, unlike the AMD
drivers for my laptop.
Added ChunkGrid, which can hold multiple chunks positioned around the
player.

Lost picking functionality in the process, as now we need to
handle picking between chunks, but the old picking was a hack
anyway.

Sometime soon some decisions will need to be made with regards to
holding positions in the world and in the chunk. For now, everything
is glm::vecx, which is internally floats. Those are good for the GPU,
but for handling large worlds may give precision issues. We probably
want glm::vecxd for game logic to avoid precision issues at moderate
sized worlds (10k-100k). However, that won't help rendering issues
which should also occur in that range, though they probably aren't
obvious until the next order of magnitude (100k-1M).