[MNT]: Color normalization in GraphicsContextBase #31105

Summary

We have

class GraphicsContextBase:

    def set_foreground(self, fg, isRGBA=False):
        """
        Set the foreground color.

        Parameters
        ----------
        fg : :mpltype:`color`
        isRGBA : bool
            If *fg* is known to be an ``(r, g, b, a)`` tuple, *isRGBA* can be
            set to True to improve performance.
        """

It feels unnecessary and like a mental burden to handle unnormalized colors the GraphicsContext. Flexible color names is primarily a user-facing topic; and maybe (t.b.d.) an artist topic so that they can reproduce a color exactly as it was specified by the user (a.set.color(c); a.get_color == c). But I think the GraphicsContext should not need to bother with this. It should only need rgba information.

Possible improvement:

• Discourage isRGBA=False
• internally only use isRGBA=True
• At some point deprecatee the isRGBA argument and remove (as part of general backend API evolution).

Semi-OT: is set_foreground the right abstraction or should we rather have set_rgba() and discorage set_foreground? Related: #31094 (comment)

Proposed fix

No response