GSGrid accept objects conforming to the GSGridItem protocol.
In preparation for sweeping changes to GSGrid, added GSNewGrid.

The old class is renamed to GSOldGrid and will remain the same to preserve
current behavior while in the process of implementing GSNewGrid.
GSLightingBuffer renamed to GSMutableByteBuffer.

The class has been renamed in preparation for being used for voxel data as
well as for lighting data.
Spit GSMutableByteBuffer into GSMutableByteBuffer and GSByteBuffer.

This allows me to declare immutable buffers which will be important once I
start coding up immutable chunk objects.
Moved GSMutableByteBuffer's save/load functionality to GSByteBuffer.
For the most part, GSByteBuffer hides the fact that each element is a byte.
GSChunkVoxelData.sunlight is now immutable.
Remove GSMutableByteBuffer since nothing is using it now.
Clean up comments and API in GSByteBuffer.

-valueAtPoint: renamed to -valueAtPosition: and comments no longer indicate
(erroneously) that it interpolates values from adjacent cells.
-lightAtPoint:getter: has much looser restrictions on buffer_element_t.
Rename GSByteBuffer to GSBuffer and change element size to 16-bit.
-newBufferFromLargerRawBuffer:... uses buffer_element_t, not uint8_t
More expansion on interface of GSGridItem.
Fill in unimplemented methods of GSNewGrid.

Also, further changes to the GSGridItem protocol. API is NOT stable.
GSAppDelegate must give up its reference to _terrain on app termination.
Disable incredibly noisy logging in DEBUG builds.
Ignore an OpenGL error which causes an abort in DEBUG builds.

There is a log message to recall the error and a comment explaining what's
going on.
GSChunkStore uses GSNewGrid for geometry.
Chunk VBOs are separated from Geometry now.

BUG: on-disk caching of geometry is currently broken.
BUG: start up now takes a long time.
BUG: geometry is not regenerated properly when voxel data changes.
BUG: chunk visibility calculations are currently broken.
BUG: there is no longer a limit on the number of VBOs that may be
     generated during a single frame.
Added new class GSGridGeometry to handle invalidation of non-resident items