plugins/screencast: Don't download texture data if target size and texture size mismatch

If glGetTexImage() gets called, it can write beyond the bounds of the
target size. In long term, it would be nice to relax this check.

CCBUG: 489764

src/plugins/screencast/screencastutils.h

@@ -41,6 +41,10 @@ static GLenum closestGLType(QImage::Format format)
 
 static void doGrabTexture(GLTexture *texture, QImage *target)
 {
+    if (texture->size() != target->size()) {
+        return;
+    }
+
     const auto context = OpenGlContext::currentContext();
     const QSize size = texture->size();
     const bool invertNeeded = context->isOpenGLES() ^ (texture->contentTransform() != OutputTransform::FlipY);