More texture image changes.

More texture image changes.
1. Added ctx->Driver.ChooseTextureFormat() function.  Examines user's
   internalFormat, format, type params and returns a gl_texture_format.
2. _mesa_store_teximage[123]d() calls ctx->Driver.ChooseTextureFormat(),
   allocates storage and transfers the image into the desired format.
3. _mesa_transfer_teximage() now takes a gl_texture_format to describe
   the destination format.  Any combination of input format/type and
   output gl_texture_format is accepted.  Uses optimized _mesa_convert_-
   texsubimage[123]d() functions when possible.
3. DRI driver's TexImage[123]D functions should be a lot simpler now.