Optimize background color and underline shapes

[JordanMartinez]

Only uses 1 shape for a value (background, underline) shared between multiple consecutive TextExt segments.

This provides the needed support to quickly implement #346 without the adjacent segments issue.

[JordanMartinez]

Tested via:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.stage.Stage;
import org.fxmisc.flowless.VirtualizedScrollPane;
import org.fxmisc.richtext.InlineCssTextArea;

public class ConsecutiveDemo extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    InlineCssTextArea area;

    @Override
    public void start(Stage primaryStage) {
        area = new InlineCssTextArea();
        area.replaceText("Some long line of text");
        area.setStyle("-fx-font-size: 32pt;");

        String blueBackground = "-rtfx-background-color: blue;";
        String greenBackground = "-rtfx-background-color: green;";
        String narrowUnderline = "-rtfx-underline-width: 2;";
        String redUnderline = "-rtfx-underline-color: red;";
        String dashedUnderline =  "-rtfx-underline-dash-array: 8;";
        String wideUnderline = "-rtfx-underline-width: 4;";
        setStyles(
                narrowUnderline,    // shouldn't produce an underline, no color specified
                blueBackground + redUnderline, // shouldn't produce an underline (no width specified)
                blueBackground + redUnderline + wideUnderline,
                greenBackground + redUnderline + wideUnderline,
                greenBackground + redUnderline + narrowUnderline + dashedUnderline,
                blueBackground + redUnderline + narrowUnderline + dashedUnderline
                
                /* 
                    Expected Shapes & Total Count:
                        background color shapes: blue, green, blue           = total of 3
                        underline shapes:        red wide, red narrow dashed = total of 2
                 */
        );

        VirtualizedScrollPane<InlineCssTextArea> vsPane = new VirtualizedScrollPane<>(area);

        Scene scene = new Scene(vsPane, 500, 500);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void setStyles(String... styles) {
        int index = 0;
        for (String s : styles) {
            area.setStyle(index, index + 1, s);
            index++;
        }
    }
}