fix handling of textures with a base internal format that does not have all four...

fix handling of textures with a base internal format that does not have all four rgba values set for radeon and r200 (discovered with a modified glean pixelFormats test, noone ever noticed in over 2 years). For radeon, use hw format I8 as previously, and change tex env to make the correct default values appear for both GL_ALPHA and GL_LUMINANCE textures. For r200, which supports GL_LUMINANCE just fine,  use the AL88 hw format for GL_ALPHA textures, since it seems like it's probably not worth the effort to fix up the texture environment (certainly complicated in case of ATI_fragment_shader programs).