Move canvas-based parsing to Color.ts lib

This removed the allowTransparency code, it didn't actually do much
before, just whether the 'advanced parsing' would allow transparency.
Since we already do the common formats, I'm not even sure what other
format it could be.

addons/xterm-addon-serialize/src/SerializeAddon.test.ts

@@ -78,7 +78,7 @@ describe('xterm-addon-serialize', () => {
     terminal.loadAddon(serializeAddon);
 
     selectionService = new TestSelectionService((terminal as any)._core._bufferService);
-    cm = new ColorManager(document, false);
+    cm = new ColorManager();
     (terminal as any)._core._colorManager = cm;
     (terminal as any)._core._selectionService = selectionService;
   });