formatters/img: use new API for bounding box for Pillow 9.2

Something like

id id2("){ ... }");

is no longer wrongly recognized as a "function"

id id2(") {
  ...
}
");

As the difference in the tests shows, this has the unfortunate side
effect that we no longer highlight something like

int f(param="default");

as a function declaration, but it is hard to imagine another way to
fix this (cf. “most vexing parse” problem).

Fixes #2207

\\\\|\\. is like \\. but causes catastrophic backtracking inside (...)* .

Very sorry about this!