MarkupText: Use Cairo to handle gradients

Nobody has claimed this yet.

## Enhancement proposal

When using gradients in `MarkupText` we have Pango render the text to an SVG. We then load the SVG into a `SVGMobject` and apply the gradient to the characters. This has two major drawbacks:

- the gradient is done on a per-character basis, i.e. every character has one solid color; the quality of the gradient therefore depends largely on the length of the text to which it is applied
- with ligatures, the number of paths does not correspond to the number of characters, so the user will have to specify an offset to compensate for this, e.g. `fl` counts for two characters but might be rendered as a single glyph
- similar problems arise with underline, double underline, overline and strikethrough, because those lines will also be sub-paths of the `SVGMobject`

We will have to be careful w.r.t. the `Write` animation. Currently, even with gradients every character has its own color. If the character is itself colored with a gradient, what color is to be used for the stroke?