Add options to sort the palette by RGBA channels

src/app/ui/color_bar.cpp

@@ -348,6 +348,10 @@ void ColorBar::onPaletteButtonClick()
         sat("Sort by Saturation"),
         bri("Sort by Brightness"),
         lum("Sort by Luminance"),
+        red("Sort by Red"),
+        grn("Sort by Green"),
+        blu("Sort by Blue"),
+        alp("Sort by Alpha"),
         asc("Ascending"),
         des("Descending");
       menu.addChild(&rev);
@@ -358,6 +362,11 @@ void ColorBar::onPaletteButtonClick()
       menu.addChild(&bri);
       menu.addChild(&lum);
       menu.addChild(new ui::MenuSeparator);
+      menu.addChild(&red);
+      menu.addChild(&grn);
+      menu.addChild(&blu);
+      menu.addChild(&alp);
+      menu.addChild(new ui::MenuSeparator);
       menu.addChild(&asc);
       menu.addChild(&des);
 
@@ -370,6 +379,10 @@ void ColorBar::onPaletteButtonClick()
       sat.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::SATURATION));
       bri.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::VALUE));
       lum.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::LUMA));
+      red.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::RED));
+      grn.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::GREEN));
+      blu.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::BLUE));
+      alp.Click.connect(Bind<void>(&ColorBar::onSortBy, this, SortPaletteBy::ALPHA));
       asc.Click.connect(Bind<void>(&ColorBar::setAscending, this, true));
       des.Click.connect(Bind<void>(&ColorBar::setAscending, this, false));
 

src/doc/sort_palette.cpp

@@ -61,6 +61,11 @@ struct PalEntryWithIndexPredicate {
         value2 = rgba_getb(c2);
         break;
 
+      case SortPaletteBy::ALPHA:
+        value1 = rgba_geta(c1);
+        value2 = rgba_geta(c2);
+        break;
+
       case SortPaletteBy::HUE:
       case SortPaletteBy::SATURATION:
       case SortPaletteBy::VALUE: {

src/doc/sort_palette.h

@@ -17,6 +17,7 @@ namespace doc {
     RED,
     GREEN,
     BLUE,
+    ALPHA,
     HUE,
     SATURATION,
     VALUE,