We were setting it to whatever value from {1,2,4,8} the user requested
and otherwise ignoring it.  There were two problems there:

1) Validation ignored it, so GL could read outside of the user's array
   in TexImage() or TexSubImage() if the aligment was greater than
   cpp.

2) TexImage()/TexSubImage() from image/canvas sources wasn't packing
   its data according to the unpack alignment.

To fix this, start tracking the user-requested alignment in the DOM
side of the context.  Set the GL's alignment to 1 for image/canvas
sources or the user's value for array sources, and pass the user's
alignment in to validation so that it can figure out the correct size
of image that the GL will ready.