[chiragbitoni]

Resolves #986

When a box annotation has negative or zero dimensions (e.g. inverted coordinates or zero width/height), Math.min(width, height) / 2 evaluates to non-positive or negative values. When passed to clampAll, this could produce a negative radius, causing canvas arc() calls to throw an IndexSizeError in browsers.

This change:

1. Returns early from drawBox if width <= 0 or height <= 0.
2. Clamps the radius upper bound with Math.max(0, Math.min(width, height) / 2) to guarantee a non-negative radius.
3. Adds tests in test/specs/box.spec.js asserting that drawing boxes with zero or negative dimensions does not throw.