Improve formatting of badly nested font blocks.

The basic idea is to already pop the font at the end marker
instead of allowing it to linger until the final end of the block.

This requires a few preliminaries:
* For each block, save a pointer to the previous font
  to be used in case the block breaks another and gets extended.
* That requires making node information writable during rendering.
* Now fonts may get popped in the wrong order; hence, after the stack
  has already been rewound further by some block that began earlier,
  ignore popping a font that was put on the stack later.
* To be able to exploit all this for font blocks, tie processing
  to their body, not their block, which is more logical anyway.

Triggered by florian@ reporting vaguely similar issues with list blocks.

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