Expose API to clear texture atlas

Using canvases on Electron (and plain Chromium I believe), when resuming
from sleep the graphics card may release the texture which results in
the terminal display and char atlas being corrupt. This API allows the
consumer to clear the texture atlas right after resuming, atlas warm up
is not done to simplify this process because the fact that atlases are
shared between multiple terminals complicates this.

Related microsoft/vscode#69665

addons/xterm-addon-webgl/src/atlas/WebglCharAtlas.ts

@@ -101,7 +101,7 @@ export class WebglCharAtlas implements IDisposable {
     }
   }
 
-  protected _doWarmUp(): void {
+  private _doWarmUp(): void {
     // Pre-fill with ASCII 33-126
     for (let i = 33; i < 126; i++) {
       const rasterizedGlyph = this._drawToCache(i, DEFAULT_COLOR, DEFAULT_COLOR);
@@ -115,17 +115,26 @@ export class WebglCharAtlas implements IDisposable {
 
   public beginFrame(): boolean {
     if (this._currentRowY > TEXTURE_CAPACITY) {
-      this._cacheCtx.clearRect(0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT);
-      this._cacheMap = {};
-      this._currentRowHeight = 0;
-      this._currentRowX = 0;
-      this._currentRowY = 0;
-      this._doWarmUp();
+      this.clearTexture();
+      this.warmUp();
       return true;
     }
     return false;
   }
 
+  public clearTexture(): void {
+    if (this._currentRowX === 0 && this._currentRowY === 0) {
+      return;
+    }
+    this._cacheCtx.clearRect(0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT);
+    this._cacheMap = {};
+    this._cacheMapCombined = {};
+    this._currentRowHeight = 0;
+    this._currentRowX = 0;
+    this._currentRowY = 0;
+    this._didWarmUp = false;
+  }
+
   public getRasterizedGlyphCombinedChar(chars: string, bg: number, fg: number): IRasterizedGlyph {
     let rasterizedGlyphSet = this._cacheMapCombined[chars];
     if (!rasterizedGlyphSet) {