Invalid operation when using depth texture

[ThibaultLejemble]

When I attach a depth texture to a render pass I get an invalid operation OpenGL error (invalid operation) during operation "glDrawBuffers((GLsizei) draw_buffers.size(), draw_buffers.data())"! from this line https://github.com/mitsuba-renderer/nanogui/blob/2ee903c96480d4aee54542ea3c340c13cc06dc32/src/renderpass_gl.cpp#L93. And then I get the critical error RenderPass::RenderPass(): framebuffer is marked as incomplete: incomplete attachment.

I am using the example1.cpp file, where I added the following lines

        m_depth_tex = new Texture(
            Texture::PixelFormat::Depth,
            Texture::ComponentFormat::Float32,
            m_size);

        m_render_pass = new RenderPass({ this }, m_depth_tex);

If if replace GL_BACK_LEFT by attachment_id here https://github.com/mitsuba-renderer/nanogui/blob/2ee903c96480d4aee54542ea3c340c13cc06dc32/src/renderpass_gl.cpp#L67 the invalid operation of glDrawBuffers is gone, but there is still the critical error of incomplete attachment.

I am using OpenGL on ubuntu.