Arrow heads of dative bonds are different sizes

[DavidACosgrove]

Describe the bug
The width of the arrow head of a dative bond is a function of the length of the line. This means that sometimes in the same drawing they are different sizes, as below. The separation between lines in a double bond is now a fixed number of pixels, and it should be the same for arrowheads as well, IMO.

To Reproduce
Steps to reproduce the behavior, feel free to paste a bit of Python in here.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Configuration (please complete the following information):

• RDKit version: master branch built from scratch
• OS: macOS 12.6.2
• Python version (if relevant):
• Are you using conda?
• If you are using conda, which channel did you install the rdkit from?
• If you are not using conda: how did you install the RDKit?

Additional context
Fix on its way