Fix two files that do not parse

[Lumen951]

Reading through the repository to follow the inference path end to end, I could not
get the package to import from a clean checkout. Two files on master do not compile,
and between them they make most of the package unimportable.

ffn/training/tracker.py

9af8e87 wrapped the font used to annotate the eval image summaries in a try/except,
but left the call in the try body commented out, so the block is empty and the
module no longer parses. import ffn.inference.inference raises IndentationError
because of it, through inference -> movement -> training.examples -> training.tracker.

The except clause catches (IOError, ValueError), which is the documented exception
contract of PIL.ImageFont.truetype (OSError if the font file cannot be read,
ValueError if the size is not a positive number), so the commented-out line was
loading a specific font file. No font file is present in the repository and its path
and size cannot be recovered from it, so I kept the fallback that the except branch
already used. ImageDraw.text calls ImageFont.load_default() when it is handed no
font, which is exactly what the except branch passed, so the rendered summaries are
unchanged: both variants produce a byte-identical PNG.

ffn/input/volume.py

8c7fcb6 renamed config.sampling.bbox to config.sampling.bounding_boxes, and in
sample_coordinates the rename was made without re-indenting the block below it,
leaving if config.sampling.bounding_boxes: with no body and a 4-space indent inside
an otherwise 2-space file. The second occurrence of the same check, further down the
file, was updated correctly.

Verification

With both changes import ffn.inference.inference succeeds, and
ffn.inference.run_inference becomes importable. Of the 62 modules in the package, 46
now import, against 38 before; the rest fail on their own missing optional
dependencies. ffn/training/tracker_test.py passes.

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