Implement alpha buffer copy for SwapBuffers().

Nicolai writes:
When the pixmap pixel format has no alpha channel, the x11 driver
(software rendering) adds a wrapped alpha channel on request.

During SwapBuffers, this alpha channel is not copied from back to
front, which means that the front buffer doesn't really contain the
contents that the back buffer previously contained.

A subsequent glReadPixels from the front buffer will return an
incorrect result. The following patch attempts to fix this.

2 files changed, 8 insertions, 5 deletions

diff --git a/src/mesa/drivers/x11/xm_api.c b/src/mesa/drivers/x11/xm_api.c
index b513dc8d40a..24b19d8eb5a 100644
--- a/src/mesa/drivers/x11/xm_api.c
+++ b/src/mesa/drivers/x11/xm_api.c
@@ -363,7 +363,6 @@ static XMesaBuffer
 create_xmesa_buffer(XMesaDrawable d, BufferType type,
                     XMesaVisual vis, XMesaColormap cmap)
 {
-   GLboolean swAlpha;
    XMesaBuffer b;
 
    ASSERT(type == WINDOW || type == PIXMAP || type == PBUFFER);
@@ -421,10 +420,10 @@ create_xmesa_buffer(XMesaDrawable d, BufferType type,
       /* Visual has alpha, but pixel format doesn't support it.
        * We'll use an alpha renderbuffer wrapper.
        */
-      swAlpha = GL_TRUE;
+      b->swAlpha = GL_TRUE;
    }
    else {
-      swAlpha = GL_FALSE;
+      b->swAlpha = GL_FALSE;
    }
 
    /*
@@ -435,9 +434,9 @@ create_xmesa_buffer(XMesaDrawable d, BufferType type,
                                 vis->mesa_visual.haveDepthBuffer,
                                 vis->mesa_visual.haveStencilBuffer,
                                 vis->mesa_visual.haveAccumBuffer,
-                                swAlpha,
+                                b->swAlpha,
                                 vis->mesa_visual.numAuxBuffers > 0 );
-   
+
    /* insert buffer into linked list */
    b->Next = XMesaBufferList;
    XMesaBufferList = b;
@@ -2211,6 +2210,9 @@ void XMesaSwapBuffers( XMesaBuffer b )
 		      );
          /*_glthread_UNLOCK_MUTEX(_xmesa_lock);*/
       }
+
+      if (b->swAlpha)
+         _mesa_copy_soft_alpha_renderbuffers(ctx, &b->mesa_buffer);
    }
 #if !defined(XFree86Server)
    XSync( b->xm_visual->display, False );
diff --git a/src/mesa/drivers/x11/xmesaP.h b/src/mesa/drivers/x11/xmesaP.h
index 5516031ffe7..85cb6b66363 100644
--- a/src/mesa/drivers/x11/xmesaP.h
+++ b/src/mesa/drivers/x11/xmesaP.h
@@ -224,6 +224,7 @@ struct xmesa_buffer {
    GLint db_mode;		/* 0 = single buffered */
 				/* BACK_PIXMAP = use Pixmap for back buffer */
 				/* BACK_XIMAGE = use XImage for back buffer */
+   GLboolean swAlpha;
 
    GLuint shm;			/* X Shared Memory extension status:	*/
 				/*    0 = not available			*/