Fix texture format selection. ChooseTextureFormat is supposed to select the

Fix texture format selection.  ChooseTextureFormat is supposed to select the
hardware format of the texture, and SetTexImages is supposed to use the
format selected by ChooseTextureFormat.  However, both routines were making
their choices based on the texture's BaseFormat.  This is wrong.
ChooseTextureFormat uses BaseFormat and SetTexImages uses
TexFormat->MesaFormat.

Once SetTexImages was fixed to use the right format values,
ChooseTextureFormat was cleaned up.  It now uses the few available texture
formats supported by the i810 in a smarter way.  This should improve the
quality of LUMINANCE, LUMINANCE_ALPHA, and INTENSITY textures.

I tested this by cycling through all the texture formats in demos/texenv and
tests/yuvsquare.