refactor(renderer): extract artboard checkerboard drawing

renderer/clear-rect.ts

@@ -0,0 +1,7 @@
+import type { RgbaColor, ScreenRect, WebGlRendererContext } from "./types";
+
+export function clearScreenRect(context: WebGlRendererContext, rect: ScreenRect, color: RgbaColor) {
+  context.gl.scissor(rect.x, context.canvas.height - rect.y - rect.h, rect.w, rect.h);
+  context.gl.clearColor(...color);
+  context.gl.clear(context.gl.COLOR_BUFFER_BIT);
+}