Implement partial support for rgba() color

Partially handles inbox#1195 (closed) . Now only rgba(..., 1) is handled. (previously it was not handled at all)

Things like rgba(0, 0, 0, 0.5) remains unhandled. (It would require a larger refactoring to propagate the alpha value)

Also use some macro to avoid code duplication. (I can refactor to use inline lambda if needed, but then exception would be needed to substitute for return)