Skip drawing whitespace from DynamicCharAtlas

When we remove the background from a glyph, we can keep track of if we
made all the pixels fully transparent. If we did, we can just skip
future calls to _drawFromCache, since it would be a noop anyways.

src/shared/atlas/CharAtlasGenerator.ts

@@ -108,8 +108,10 @@ export function generateStaticCharAtlasTexture(context: Window, canvasFactory: (
 
 /**
  * Makes a partiicular rgb color in an ImageData completely transparent.
+ * @returns True if the result is "empty", meaning all pixels are fully transparent.
  */
-export function clearColor(imageData: ImageData, color: IColor): void {
+export function clearColor(imageData: ImageData, color: IColor): boolean {
+  let isEmpty = true;
   const r = color.rgba >>> 24;
   const g = color.rgba >>> 16 & 0xFF;
   const b = color.rgba >>> 8 & 0xFF;
@@ -118,8 +120,11 @@ export function clearColor(imageData: ImageData, color: IColor): void {
         imageData.data[offset + 1] === g &&
         imageData.data[offset + 2] === b) {
       imageData.data[offset + 3] = 0;
+    } else {
+      isEmpty = false;
     }
   }
+  return isEmpty;
 }
 
 function getFont(fontWeight: FontWeight, config: ICharAtlasConfig): string {