In functions texstore_rgba_fxt1 and texstore_rgb_fxt1(texcompress_fxt1.c), calling 
_mesa_compressed_image_address with mesaFormat set to
GL_COMPRESSED_RGB_FXT1_3DFX or GL_COMPRESSED_RGBA_FXT1_3DFX. From the context,
it is a mistake. Here it should pass a Mesa internal format, or it will trigger
a mesa error.

    107    dst = _mesa_compressed_image_address(dstXoffset, dstYoffset, 0,
    108                                         GL_COMPRESSED_RGB_FXT1_3DFX,
                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    109                                         texWidth, (GLubyte *) dstAddr);