backends/drm: Mark DMA bufs as mirrored when exporting textures

When streaming direct scan out we take the layers texture directly.
When setting the contentTransform we discard the mirrorY transform that
we know is set on DMA bufs. This results in recording being upside-down.

src/backends/drm/drm_egl_layer.cpp

@@ -83,7 +83,7 @@ std::shared_ptr<GLTexture> EglGbmLayer::texture() const
 {
     if (m_scanoutBuffer) {
         const auto ret = m_surface.eglBackend()->importDmaBufAsTexture(*m_scanoutBuffer->buffer()->dmabufAttributes());
-        ret->setContentTransform(drmToTextureRotation(m_pipeline));
+        ret->setContentTransform(drmToTextureRotation(m_pipeline) | TextureTransform::MirrorY);
         return ret;
     } else {
         return m_surface.texture();