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);
- 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)).

Edited Jan 18, 2022 by Ronald S. Bultje

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