* ASTC6x6 support and more comment abount bpp usage in engine * fix log output * Comment astc compression tool download url * Add command line comment for astcenc tool [skip travis][skip appveyor] * Add astc_6x6 test image[skip appveyor][skip travis] * astc_6x6 mtl * Refine log info[skip appveyor][skip travis] * Empty line[skip appveyor][skip travis]