fix(Metal): always render explicit background colors fully opaque

This fixes a regression introduced by the rework of this area before
during the color space changes. It seems like the original intent of
this code was the behavior it regressed to, but it turns out to be
better like this.

src/renderer/Metal.zig

@@ -2672,9 +2672,8 @@ fn rebuildCells(
                     // Cells that are reversed should be fully opaque.
                     if (style.flags.inverse) break :bg_alpha default;
 
-                    // Cells that have an explicit bg color, which does not
-                    // match the current surface bg, should be fully opaque.
-                    if (bg != null and !rgb.eql(self.background_color orelse self.default_background_color)) {
+                    // Cells that have an explicit bg color should be fully opaque.
+                    if (bg_style != null) {
                         break :bg_alpha default;
                     }