This fixes a couple of tests doing RGBA/ubyte image uploads with
flipping.  Other tests with flipping get their expectations changed,
because some other feature is missing (premultiplication or
ImageData/canvas format conversion)

The code was returning RGBA8 data from the non-raw sources (HTML
canvas elements, JS ImageData, etc.), but we then validated and passed
that rgba8 data as if it was whatever format/datatype was specified in
TexImage2D/TexSubImage2D, so the pixels would come out as garbage.

It would seem like we could just rewrite the passed in format/datatype
for the TexImage call to be RGBA/UNSIGNED_BYTE, but that would leave
incorrect levels of precision if the internalformat didn't match the
format/datatype (and older desktop implementations often ignore the
internalformat in choosing their internal format, anyway).

Now the affected testcase only fails due to unpack alignment.

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.