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).