backends/drm: disable the hardware cursor with color management

Blending needs to happen in linear space, which is not the case if the
inverse EOTF and VCGT are applied before blending

src/backends/drm/drm_egl_cursor_layer.cpp

@@ -47,6 +47,10 @@ EglGbmCursorLayer::EglGbmCursorLayer(EglGbmBackend *eglBackend, DrmPipeline *pip
 
 std::optional<OutputLayerBeginFrameInfo> EglGbmCursorLayer::beginFrame()
 {
+    if (m_pipeline->output()->needsColormanagement()) {
+        // TODO for hardware cursors to work with color management, KWin needs to offload post-blending color management steps to KMS
+        return std::nullopt;
+    }
     return m_surface.startRendering(m_pipeline->gpu()->cursorSize(), drmToTextureRotation(m_pipeline) | TextureTransform::MirrorY, m_pipeline->cursorFormats(), m_pipeline->colorDescription(), m_pipeline->output()->channelFactors(), m_pipeline->output()->needsColormanagement());
 }