Rename Matrix#setIdentity() to #reset() and document it.
Fix TODO in Matrix#scale().
Update JsDoc.
More work on Matrix decomposition.

Have Matrix#decompose() return null if decomposition is not possible, separate Matrix#translation from decomposition code as it's always defined, and document methods and properties.
Restructure Matrix code sequence.
Adapt to recent changes to Matrix#decompose() in SvgExport code.