Following #1478, a bug was found related to transparent backgrounds. As Excalidraw only supports `transparent` as a valid transparent color, this commits generalizes the use of canvas to normalize color values.
It changes a few details:
- `rgba()` or `hsla()` syntaxes are not accepted anymore
- pasting values goes through the same normalization step, avoiding invalid values
- color validation is not regex-based anymore
- any CSS-valid black color is now accepted (e.g. previously, `rgb(0,0,0)` was rejected)
