Draw molecule on specific canvas area using drawMolecule

We at Datagrok are [using openchemlib for rendering molecules](https://github.com/datagrok-ai/public/blob/master/packages/Chem/src/ocl_cell_renderer.ts). It is a common task to render multiple molecules simultaneously, but currently it is necessary to use separate canvas element to be able to draw each molecule in a specific area of a larger canvas meaning excessive resources usage. It would be very useful to be able to draw a molecule in a specific canvas area when using `drawMolecule` by providing such parameters as _xy_ coordinates of top left vertex of the area rectangle as well as its _width_ and _height_.

Start with the drawMolecule implementation and the Datagrok ocl_cell_renderer.ts usage linked in the issue. Trace how canvas coordinates and molecule dimensions are currently handled, then define support for a rectangular drawing area using its top-left coordinates, width, and height while preserving existing calls. Done means multiple molecules can render in separate areas of one canvas.

Written by the indexing model from the issue text.