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.

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 );

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			*/

src/mesa/main/renderbuffer.c

@@ -1435,6 +1435,17 @@ put_mono_values_alpha8(GLcontext *ctx, struct gl_renderbuffer *arb,
 }
 
 
+static void
+copy_buffer_alpha8(struct gl_renderbuffer* dst, struct gl_renderbuffer* src)
+{
+   ASSERT(dst->_ActualFormat == GL_ALPHA8);
+   ASSERT(src->_ActualFormat == GL_ALPHA8);
+   ASSERT(dst->Width == src->Width);
+   ASSERT(dst->Height == src->Height);
+
+   _mesa_memcpy(dst->Data, src->Data, dst->Width * dst->Height * sizeof(GLubyte));
+}
+
 
 /**********************************************************************/
 /**********************************************************************/
@@ -1765,6 +1776,27 @@ _mesa_add_alpha_renderbuffers(GLcontext *ctx, struct gl_framebuffer *fb,
 }
 
 
+/**
+ * For framebuffers that use a software alpha channel wrapper
+ * created by _mesa_add_alpha_renderbuffer or _mesa_add_soft_renderbuffers,
+ * copy the back buffer alpha channel into the front buffer alpha channel.
+ */
+void
+_mesa_copy_soft_alpha_renderbuffers(GLcontext *ctx, struct gl_framebuffer *fb)
+{
+   if (fb->Attachment[BUFFER_FRONT_LEFT].Renderbuffer &&
+       fb->Attachment[BUFFER_BACK_LEFT].Renderbuffer)
+      copy_buffer_alpha8(fb->Attachment[BUFFER_FRONT_LEFT].Renderbuffer,
+                         fb->Attachment[BUFFER_BACK_LEFT].Renderbuffer);
+
+
+   if (fb->Attachment[BUFFER_FRONT_RIGHT].Renderbuffer &&
+       fb->Attachment[BUFFER_BACK_RIGHT].Renderbuffer)
+      copy_buffer_alpha8(fb->Attachment[BUFFER_FRONT_RIGHT].Renderbuffer,
+                         fb->Attachment[BUFFER_BACK_RIGHT].Renderbuffer);
+}
+
+
 /**
  * Add a software-based depth renderbuffer to the given framebuffer.
  * This is a helper routine for device drivers when creating a

src/mesa/main/renderbuffer.h

@@ -64,6 +64,9 @@ _mesa_add_alpha_renderbuffers(GLcontext *ctx, struct gl_framebuffer *fb,
                               GLboolean frontLeft, GLboolean backLeft,
                               GLboolean frontRight, GLboolean backRight);
 
+extern void
+_mesa_copy_soft_alpha_renderbuffers(GLcontext *ctx, struct gl_framebuffer *fb);
+
 extern GLboolean
 _mesa_add_depth_renderbuffer(GLcontext *ctx, struct gl_framebuffer *fb,
                              GLuint depthBits);