From 0fc1142ca229dcc40d7fd29ee3eccc633fe3955b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Gr=C3=A4=C3=9Flin?= <mgraesslin@kde.org>
Date: Thu, 8 Oct 2015 15:54:24 +0200
Subject: [PATCH] Allow OpenGLBackend to hold a surfaceless context

For a virtual rendering backend a surfaceless context is needed. Such
a context may not call glDrawBuffer as that results in a GL error.

This change allows the OpenGLBackend to announce that it's context is
surfaceless.
---
 scene_opengl.cpp |  4 +++-
 scene_opengl.h   | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/scene_opengl.cpp b/scene_opengl.cpp
index 8b3be71d50..13714e3634 100644
--- a/scene_opengl.cpp
+++ b/scene_opengl.cpp
@@ -405,7 +405,9 @@ SceneOpenGL::SceneOpenGL(OpenGLBackend *backend, QObject *parent)
         return;
     }
 #ifndef KWIN_HAVE_OPENGLES
-    glDrawBuffer(GL_BACK);
+    if (!m_backend->isSurfaceLessContext()) {
+        glDrawBuffer(GL_BACK);
+    }
 #endif
 
     m_debug = qstrcmp(qgetenv("KWIN_GL_DEBUG"), "1") == 0;
diff --git a/scene_opengl.h b/scene_opengl.h
index 5a697bdfdc..8f7c3d3ca3 100644
--- a/scene_opengl.h
+++ b/scene_opengl.h
@@ -482,6 +482,13 @@ public:
         return m_haveBufferAge;
     }
 
+    /**
+     * @returns whether the context is surfaceless
+     **/
+    bool isSurfaceLessContext() const {
+        return m_surfaceLessContext;
+    }
+
     /**
      * Returns the damage that has accumulated since a buffer of the given age was presented.
      */
@@ -561,6 +568,13 @@ protected:
         m_renderTimer.start();
     }
 
+    /**
+     * @param set whether the context is surface less
+     **/
+    void setSurfaceLessContext(bool set) {
+        m_surfaceLessContext = set;
+    }
+
     SwapProfiler m_swapProfiler;
 
 private:
@@ -596,6 +610,7 @@ private:
      * @brief Timer to measure how long a frame renders.
      **/
     QElapsedTimer m_renderTimer;
+    bool m_surfaceLessContext = false;
 };
 
 class SceneOpenGLDecorationRenderer : public Decoration::Renderer