Add bg+powerline background blend behavior

When a with bg is selected, the cell's regular background will be blended
with 50% opacity selectionBackground in order to retain the bg color info
as well as getting the color to change and getting decent contrast.

This commit also fixes a terrible, nasty bug where the standalone shared
renderer tests were causing the terminal to open multiple times and not
get the WebGL addon activated the second time. So some tests were actually
testing the DOM renderer :o

Fixes #4918

src/common/Color.test.ts

@@ -271,6 +271,27 @@ describe('Color', () => {
   });
 
   describe('rgba', () => {
+    describe('blend', () => {
+      it('should blend colors based on the alpha channel', () => {
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF00), 0x000000FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF10), 0x101010FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF20), 0x202020FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF30), 0x303030FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF40), 0x404040FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF50), 0x505050FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF60), 0x606060FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF70), 0x707070FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF80), 0x808080FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFF90), 0x909090FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFA0), 0xA0A0A0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFB0), 0xB0B0B0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFC0), 0xC0C0C0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFD0), 0xD0D0D0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFE0), 0xE0E0E0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFF0), 0xF0F0F0FF);
+        assert.deepEqual(rgba.blend(0x000000FF, 0xFFFFFFFF), 0xFFFFFFFF);
+      });
+    });
     describe('ensureContrastRatio', () => {
       it('should return undefined if the color already meets the contrast ratio (black bg)', () => {
         assert.equal(rgba.ensureContrastRatio(0x000000ff, 0x606060ff, 1), undefined);

src/common/Color.ts

@@ -245,6 +245,23 @@ export namespace rgb {
  * Helper functions where the source type is "rgba" (number: 0xrrggbbaa).
  */
 export namespace rgba {
+  export function blend(bg: number, fg: number): number {
+    $a = (fg & 0xFF) / 0xFF;
+    if ($a === 1) {
+      return fg;
+    }
+    const fgR = (fg >> 24) & 0xFF;
+    const fgG = (fg >> 16) & 0xFF;
+    const fgB = (fg >> 8) & 0xFF;
+    const bgR = (bg >> 24) & 0xFF;
+    const bgG = (bg >> 16) & 0xFF;
+    const bgB = (bg >> 8) & 0xFF;
+    $r = bgR + Math.round((fgR - bgR) * $a);
+    $g = bgG + Math.round((fgG - bgG) * $a);
+    $b = bgB + Math.round((fgB - bgB) * $a);
+    return channels.toRgba($r, $g, $b);
+  }
+
   /**
    * Given a foreground color and a background color, either increase or reduce the luminance of the
    * foreground color until the specified contrast ratio is met. If pure white or black is hit