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

1 files changed, 15 insertions, 0 deletions

diff --git a/vcl/opengl/framebuffer.cxx b/vcl/opengl/framebuffer.cxx
index c009ccb2601e..464662d31709 100644
--- a/vcl/opengl/framebuffer.cxx
+++ b/vcl/opengl/framebuffer.cxx
@@ -72,6 +72,16 @@ void OpenGLFramebuffer::AttachTexture( const OpenGLTexture& rTexture )
     mnHeight = rTexture.GetHeight();
     glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, mnAttachedTexture, 0);
     CHECK_GL_ERROR();
+
+    GLuint nStencil = rTexture.StencilId();
+    if( nStencil )
+    {
+        VCL_GL_INFO( "Attaching stencil " << nStencil << " to framebuffer " << (int)mnId );
+        glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT,
+                                   GL_RENDERBUFFER, nStencil );
+        CHECK_GL_ERROR();
+    }
+
     GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
     CHECK_GL_ERROR();
     if (status != GL_FRAMEBUFFER_COMPLETE)
@@ -87,6 +97,11 @@ void OpenGLFramebuffer::DetachTexture()
         mnAttachedTexture = 0;
         glFramebufferTexture2D( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0 );
         CHECK_GL_ERROR();
+
+        // FIXME: we could make this conditional on having a stencil ?
+        glFramebufferRenderbuffer( GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT,
+                                   GL_RENDERBUFFER, 0 );
+        CHECK_GL_ERROR();
     }
 }