Pull bg rendering into an IRenderLayer

src/renderer/Interfaces.ts

@@ -0,0 +1,6 @@
+import { ITerminal } from '../Interfaces';
+
+export interface IRenderLayer {
+  resize(canvasWidth: number, canvasHeight: number, charWidth: number, charHeight: number): void;
+  render(terminal: ITerminal, startRow: number, endRow: number): void;
+}