plugins/screencast: Allocate offscreen texture in WindowScreenCastSource::render(QImage) as big as the memfd buffer

textureSize() can temporarily mismatch the target buffer size. It can be
a problem if glGetTexImage() gets called. glGetTexImage() assumes that
the provided buffer is as big as the texture. If it's not, it will write
data outside the bounds of the buffer.

BUG: 489764

src/plugins/screencast/windowscreencastsource.cpp

@@ -59,7 +59,7 @@ qreal WindowScreenCastSource::devicePixelRatio() const
 
 void WindowScreenCastSource::render(QImage *target)
 {
-    const auto offscreenTexture = GLTexture::allocate(GL_RGBA8, textureSize());
+    const auto offscreenTexture = GLTexture::allocate(GL_RGBA8, target->size());
     if (!offscreenTexture) {
         return;
     }