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

src/browser/renderer/atlas/BaseCharAtlas.ts

@@ -26,7 +26,7 @@ export abstract class BaseCharAtlas implements IDisposable {
    * 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(): void { }
+  private _doWarmUp(): void { }
 
   /**
    * Called when we start drawing a new frame.