Add new debug features

Some of this is in !842 (closed), I'll split it in individual MRs for easier merging.

• allow outputting invisible (alt-ref) frames (!1338 (merged));
• consider exporting recon.h's DEBUG_BLOCK_INFO and DEBUG_B_PIXELS as options which do not require recompilation (!1353 (closed));
  • and then obviously, printf() should be converted to dav1d_log();
• allow outputting each frame in its own .y4m or .yuv file - with optional templates (width, height, frame num; libaom uses %w, %h and %$number for that) - to assist in debugging SVC streams (!1336 (merged));
• allow selectively disabling one or more postfilters (deblock, cdef, looprestoration, filmgrain; !1341 (merged)).