A figure states its own size, and floats are placed by default

[tannevaled]

A figure this engine cannot draw still says how big it is: an EPS in its
%%BoundingBox, a PDF in its /MediaBox. Reading that costs no rendering at all
and is what makes the reserved box the right SHAPE — with only [width=…] given,
the height then follows the figure's own aspect.

Without it, a width-only placeholder came out square. Measured on 2408.08037:
every figure reserved 476.55 × 476.55 pt — nearly a whole page — twenty-one times
over, and the paper ran to 29 pages against a reference of 12.

With figures the right shape, the float placer is finally worth having on. It is
what puts a figure at the top of its page and, when it will not fit with
\textfraction of text under it, on a later page or a float page of its own —
where the inline path lets a tall figure push the text down and leave the bottom
of the page empty. GOTEX_FLOATS=0 restores the inline path.

Measured, 157 arXiv papers against tectonic

	Σ page error	median	exact	≤1 page	≤2 pages
inline (today, after #200)	616	2	21	58	83
this	586	2	21	59	93

beamer is untouched: 40 pages of error over 79 decks either way.

Neither half is worth landing alone, which is what issue #146 asked to be
shown before either shipped: the declared box without the placer measures 637 —
the extra height has nowhere to go — and the placer without the declared box 650,
because it is then placing square figures. Landing them together is the "one
block" that issue asked for.