Add new debug features

• allow outputting invisible (alt-ref) frames;
• allow outputting each frame in its own .y4m or .yuv file - with optional templates to assist in debugging SVC streams;
• allow selectively disabling one or more postfilters (deblock, cdef, looprestoration, filmgrain);
• consider exporting recon.h's DEBUG_BLOCK_INFO and DEBUG_B_PIXELS as options which do not require recompilation;
  • and then obviously, printf() should be converted to dav1d_log().