backends/drm: re-allow the hardware cursor with color management

It doesn't look wrong anymore, presumably what caused it to look wrong before
was just a bug. Blending in sRGB or PQ is still technically wrong, but it
looks okay, and that's an acceptable tradeoff to make in order to get the
responsiveness and power usage improvements the hardware cursor offers

src/backends/drm/drm_egl_cursor_layer.cpp

@@ -48,10 +48,9 @@ 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;
-    }
+    // note that this allows blending to happen in sRGB or PQ encoding.
+    // That's technically incorrect, but it looks okay and is intentionally allowed
+    // as the hardware cursor is more important than an incorrectly blended cursor edge
     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->iccProfile(), m_pipeline->output()->needsColormanagement());
 }