Be more explicit about how to get a largeop glyph

From https://w3c.github.io/mathml-core/#layout-of-operators

If the operator has the largeop property and if math-style on the element is normal, then:
Use the MathVariants table to try and find a glyph of height at least DisplayOperatorMinHeight. If none is found, fall back to the largest non-base glyph. If none is found, fall back to the layout algorithm of 3.2.1.1 Layout of .

There is a separate issue #126 about how to actually interpret DisplayOperatorMinHeight, and it seems the conclusion is that MathML Core is correct but Cambria Math font is not.

But even without that, "use the MathVariants table" is vague. First the base glyph is obtained from the "single character c". Then I'm not sure, maybe we can just shape the base glyph to block dimension DisplayOperatorMinHeight which seems to be what Chromium does or maybe we need to introduce a similar "find a display operator glyph" algorithm if it needs to be different.

Start with MathML Core's “Layout of operators” and “shape a stretchy glyph” definitions, then compare Chromium's math_operator_layout_algorithm.cc around line 131. Review issue #126's DisplayOperatorMinHeight discussion; done means the specification explicitly defines how the base glyph and MathVariants or display-operator selection are determined.