[palette_generator]Fix PaletteGenerator.fromImageProvider region not scaled to fit image size.

[akindone]

ui.Image.size is pixel size of Image, but the param size is the dp size of the widget, and region is same the dp size.

static Future<PaletteGenerator> fromImageProvider(
    ImageProvider imageProvider, {
    Size size,
    Rect region,
    int maximumColorCount,
    List<PaletteFilter> filters,
    List<PaletteTarget> targets,
    Duration timeout = const Duration(seconds: 15),
  }) 

[gspencergoog]

Thanks for the PR!

Indeed, this looks like it needed to be fixed.