Enforce half minimum contrast ratio for dim text

Provided MCR is not too high, this ensures dim text differs from normal
text and has a reasonable difference in contrast.

Fixes #4262

src/browser/renderer/shared/CharAtlasUtils.ts

@@ -24,7 +24,8 @@ export function generateConfig(deviceCellWidth: number, deviceCellHeight: number
     // For the static char atlas, we only use the first 16 colors, but we need all 256 for the
     // dynamic character atlas.
     ansi: colors.ansi.slice(),
-    contrastCache: colors.contrastCache
+    contrastCache: colors.contrastCache,
+    halfContrastCache: colors.halfContrastCache
   };
   return {
     customGlyphs: options.customGlyphs,