[Skia] Only respect ui.Paint.dither when the colorSource is a gradient

[matanlurey]

In the Impeller backend, we only support dithering of gradients. In addition, it will be the default (and only option).

In the process of enabling dithering by default, i.e.

class Paint {
-  static bool enableDithering = false;
+  static bool enableDithering = true;
}

... we realized with internal Google testing this will now apply dithering on more than just gradients, i.e. images in the Skia backend. Since we won't support dithering of images in the Impeller backend, this PR gives a "hint" on whether the colorSource (if one is set) can be dithered by the contrived rules we've created.

[jonahwilliams]

LGTM

[flar]

Suggested changes.

(Though I really dislike the concept of "ForSuchAndSuchBackend" when describing a basic property of an object. Really, it's just identifying that a color source is some kind of gradient...)

[matanlurey]

Though I really dislike the concept of "ForSuchAndSuchBackend" when describing a basic property of an object. Really, it's just identifying that a color source is some kind of gradient...

Fair enough! I wanted to make sure I (and nobody else) felt like it was anything more than a hack for a specific backend, but I don't have a strong feeling about the name specifically.

[flar]

Should remove dead code.

[flar]

LGTM

(technically, ToSk(paint) could be moved to the conversions file. I'm not sure why that appears on its own (and I wrote this))

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