Remove use of Promises in {Base,Static}CharAtlas

Nothing actually cares about the return value, so we can avoid depending
on `Promise` by not returning a promise.

This should make this work for IE11 (though I haven't tested that).

I tried this in Chrome and Firefox to test both codepaths (ImageBitmap
and HTMLCanvasElement).

src/renderer/atlas/BaseCharAtlas.ts

@@ -6,26 +6,24 @@
 import { IGlyphIdentifier } from './Types';
 
 export default abstract class BaseCharAtlas {
-  private _didWarmUp: Promise<any>;
+  private _didWarmUp: boolean = false;
 
   /**
    * Perform any work needed to warm the cache before it can be used. May be called multiple times.
    * Implement _doWarmUp instead if you only want to get called once.
    */
-  public warmUp(): Promise<any> {
-    if (this._didWarmUp == null) {
-      this._didWarmUp = this._doWarmUp();
+  public warmUp(): void {
+    if (!this._didWarmUp) {
+      this._doWarmUp();
+      this._didWarmUp = true;
     }
-    return this._didWarmUp;
   }
 
   /**
    * Perform any work needed to warm the cache before it can be used. Used by the default
    * implementation of warmUp(), and will only be called once.
    */
-  protected _doWarmUp(): Promise<any> {
-    return Promise.resolve();
-  }
+  protected _doWarmUp(): void { }
 
   /**
    * May be called before warmUp finishes, however it is okay for the implementation to

src/renderer/atlas/StaticCharAtlas.ts

@@ -4,8 +4,7 @@
  */
 
 import { DIM_OPACITY, IGlyphIdentifier } from './Types';
-import { ICharAtlasConfig } from '../../shared/atlas/Types';
-import { CHAR_ATLAS_CELL_SPACING } from '../../shared/atlas/Types';
+import { CHAR_ATLAS_CELL_SPACING, ICharAtlasConfig } from '../../shared/atlas/Types';
 import { generateStaticCharAtlasTexture } from '../../shared/atlas/CharAtlasGenerator';
 import BaseCharAtlas from './BaseCharAtlas';
 
@@ -23,12 +22,14 @@ export default class StaticCharAtlas extends BaseCharAtlas {
     return canvas;
   }
 
-  public async _doWarmUp(): Promise<void> {
+  public _doWarmUp() {
     const result = generateStaticCharAtlasTexture(window, this._canvasFactory, this._config);
-    if (result instanceof Promise) {
-      this._texture = await result;
-    } else {
+    if (result instanceof HTMLCanvasElement) {
       this._texture = result;
+    } else {
+      result.then(texture => {
+        this._texture = texture
+      });
     }
   }