Bug 1661869 - Fix screenshotting of WebGL canvases. r=gfx-reviewers,lsalzman

For WebGL canvases with preserveDrawingBuffer:false, frontbuffer
snapshotting was not working properly.
If the frontbuffer had no user-fb associated with it,
was inexplicably calling BindDefaultFBForRead, which binds
*webgl*'s default FB, not *gl*'s default FB 0.
This caused frontbuffer snappshotting to actually snapshot the (cleared)
backbuffer instead of the visible frontbuffer contents.

Differential Revision: https://phabricator.services.mozilla.com/D126024

dom/canvas/WebGLContext.cpp

@@ -996,14 +996,7 @@ Maybe<uvec2> WebGLContext::FrontBufferSnapshotInto(
     }
   });
 
-  if (front->mFb) {
-    gl->fBindFramebuffer(fbTarget, front->mFb->mFB);
-  } else {
-    if (!BindDefaultFBForRead()) {
-      gfxCriticalError() << "BindDefaultFBForRead failed";
-      return {};
-    }
-  }
+  gl->fBindFramebuffer(fbTarget, front->mFb ? front->mFb->mFB : 0);
   if (pboWas) {
     BindBuffer(LOCAL_GL_PIXEL_PACK_BUFFER, nullptr);
   }