drm: Properly specify the yInversion of the shadow buffer

Otherwise when we render it, we do so upside down and screen sharing
looks broken.
This only happens when the shadow buffer is in use, so it's not all that
common.

src/backends/drm/shadowbuffer.cpp

@@ -40,6 +40,7 @@ ShadowBuffer::ShadowBuffer(const QSize &size, const GbmFormat &format)
 {
     m_texture.reset(new GLTexture(internalFormat(format), size));
     m_texture->setFilter(GL_NEAREST);
+    m_texture->setYInverted(true);
 
     m_renderTarget.reset(new GLRenderTarget(m_texture.data()));
     if (!m_renderTarget->valid()) {