Rename ITheme.selection to selectionBackground

This makes it consistent with selectionForeground and other theme keys and
as a result will make the API easier to work with, especially when inactive
selection is done (#3803)

addons/xterm-addon-webgl/src/WebglRenderer.ts

@@ -405,7 +405,7 @@ export class WebglRenderer extends Disposable implements IRenderer {
 
     // Apply the selection color if needed
     if (this._isCellSelected(x, y)) {
-      bgOverride = this._colors.selectionOpaque.rgba >> 8 & 0xFFFFFF;
+      bgOverride = this._colors.selectionBackgroundOpaque.rgba >> 8 & 0xFFFFFF;
       if (this._colors.selectionForeground) {
         fgOverride = this._colors.selectionForeground.rgba >> 8 & 0xFFFFFF;
       }

addons/xterm-addon-webgl/src/atlas/CharAtlasUtils.ts

@@ -21,8 +21,8 @@ export function generateConfig(scaledCellWidth: number, scaledCellHeight: number
     background: colors.background,
     cursor: NULL_COLOR,
     cursorAccent: NULL_COLOR,
-    selectionTransparent: NULL_COLOR,
-    selectionOpaque: NULL_COLOR,
+    selectionBackgroundTransparent: NULL_COLOR,
+    selectionBackgroundOpaque: NULL_COLOR,
     selectionForeground: NULL_COLOR,
     // For the static char atlas, we only use the first 16 colors, but we need all 256 for the
     // dynamic character atlas.