tdf#93529 - move to a Mac-like double-buffered OpenGL model.

This moves us to always rendering to an off-screen texture, and then
(at idle) blitting this to the screen & swapping buffers. Ideally we
should never see any rendering, or flicker again with this approach.

Several fixes are included:
+ avoid multiple OpenGL contexts being created for the same window,
created excessive flicker problems.
+ de-virtualize UseContext - which context we use is less critical.
+ kill 'mbOffscreen' distinction - all VCL rendering is offscreen.
+ implement 'doFlush' and high priority idle flushing.
+ bind stencil buffer for clipping vs. textures - fixing complex
clopping when rendering to virtual-devices, and off-screen.
+ document environment. variables.
+ use white as default background glClear color, but red or
random color for DBGUTIL.

Change-Id: I6be08595b6c8deb7e6db0dbd81308b2c97d2b4ff

3 files changed, 7 insertions, 3 deletions

diff --git a/vcl/inc/opengl/texture.hxx b/vcl/inc/opengl/texture.hxx
index f67b3348d14d..e57aa9e32306 100644
--- a/vcl/inc/opengl/texture.hxx
+++ b/vcl/inc/opengl/texture.hxx
@@ -37,6 +37,8 @@ public:
     int    mnWidth;
     int    mnHeight;
     GLenum mnFilter;
+    GLuint mnOptStencil;
+    bool   mbHasOptStencil;
 
     std::unique_ptr<std::vector<int>> mpSlotReferences;
     int mnFreeSlots;
@@ -76,7 +78,8 @@ public:
     }
 
     bool InitializeSlots(int nSlotSize);
-    int FindFreeSlot();
+    int  FindFreeSlot();
+    GLuint AddStencil();
 };
 
 class VCL_DLLPUBLIC OpenGLTexture
@@ -110,6 +113,9 @@ public:
     void            Bind();
     void            Unbind();
     void            Read( GLenum nFormat, GLenum nType, sal_uInt8* pData );
+    GLuint          AddStencil();
+    bool            HasStencil() const;
+    GLuint          StencilId() const;
 
     void            SaveToFile(const OUString& rFileName);
 
diff --git a/vcl/inc/opengl/win/gdiimpl.hxx b/vcl/inc/opengl/win/gdiimpl.hxx
index ecefede6e7ea..8102d2ef5c07 100644
--- a/vcl/inc/opengl/win/gdiimpl.hxx
+++ b/vcl/inc/opengl/win/gdiimpl.hxx
@@ -31,7 +31,6 @@ public:
 
 protected:
     virtual rtl::Reference<OpenGLContext> CreateWinContext() override;
-    virtual bool UseContext( const rtl::Reference<OpenGLContext> &pContext ) override;
 
     bool RenderTextureCombo(TextureCombo& rCombo, int nX, int nY);
 
diff --git a/vcl/inc/opengl/x11/gdiimpl.hxx b/vcl/inc/opengl/x11/gdiimpl.hxx
index f07468d6a135..eccd0ef06ae9 100644
--- a/vcl/inc/opengl/x11/gdiimpl.hxx
+++ b/vcl/inc/opengl/x11/gdiimpl.hxx
@@ -29,7 +29,6 @@ public:
 
 protected:
     virtual rtl::Reference<OpenGLContext> CreateWinContext() override;
-    virtual bool UseContext( const rtl::Reference<OpenGLContext> &pContext ) override;
 
     bool RenderPixmap(X11Pixmap* pPixmap, X11Pixmap* pMask, int nX, int nY, TextureCombo& rCombo);