Quantized mesh upsample fixes

Rather than converting all of our u and v data from shorts to doubles, perform upsampling operations, and converting back to shorts, let's work in the short number space instead.  This makes a difference in that our splitting plane is at 0.5, a number that cannot be represented in the quantized domain [0, 32767].  Because of this, vertices that should be on the splitting plane and included in both neighboring upsampled children would have only been included in the west (or south) upsampled tile before.  With this fix in place, we are also properly generating skirts, as we are correctly identifying the edgeIndices in an upsampled tile.
`midUV` is never used.