Cleanups and fixes to PixelConversions

Fixes:
  * Color conversion of 5551, 565, 4444 textures. Blue and Red were swapped.
  * Luminance alpha textures. Both components were mapped to the same 8 bits out of the 16 bit value.

  I had the bad luck of testing this change originally with a brightly
  colored abstract block game, and I didn't noticed that the red and
  blue channels were swapped. I tracked it down to a confusion on my
  part about the order of the packed components in each integer, and how
  it worked for bit-packed and non-bit-packed forms.

  This quick fix is to just swap around masks for the Red and Blue
  components for the packed-pixel types (such as
  GL_UNSIGNED_SHORT_5_6_5), and adjust the corresponding tests to go
  along with it.

  This looks inconsistent with the surrounding declarations, as the
  bit-packed forms say that red (or at least the first component) is in
  the highest bits, while the non-bit-packed forms seem to say that red
  is in the lowest bits. Both however are correct due to the way the
  values are read out of memory.

  I spent an extra day trying to make this consistent, and also
  properly handle endian swapping. However I got bogged down in the
  clean up, and I ran out of time to do it for now. I left some comments
  around it in the code, and I'll keep my work in progress patch, but I
  won't have time in the near future to do anything on it.

  Along the way, I also noticed that the intensity-alpha format
  specification was using the first byte for both values, instead of
  using the second byte for alpha as it was supposed to.

Cleanups:

  * I removed some dead declarations that I left behind before submitting.
    I had meant to take them out, and missed them.

  * I added some comments to document the RegalPixelConversions header.

  * Removed a TODO comment around using memcpy when dealing with
    conversions of <GL_RGBA, GL_UNSIGNED_BYTE> to/from the intermediate
    format. There is an opportunity to use a memcpy for some formats,
    but for the format I called out it may not be possible (depends on
    endian).