Invisible horizontal bond in svg of two-atom SMILES

[Acylation]

Sample smiles: CC and CO

Proposed changes:

Setting 'maskUnits' attribute of the mask elements to 'userSpaceOnUse'

Keyword objectBoundingBox should not be used when the geometry of the applicable element has no width or no height, such as the case of a horizontal or vertical line, even when the line has actual thickness when viewed due to having a non-zero stroke width since stroke width is ignored for bounding box calculations. When the geometry of the applicable element has no width or height and objectBoundingBox is specified, then the given effect (e.g., a gradient or a filter) will be ignored.

https://bugs.chromium.org/p/chromium/issues/detail?id=570432
https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/maskUnits