Kill gtktiledviewer's alpha channel hack.

basebmp and vcl now set the alpha channel appropriately, so no need
to do so in the viewer now.

However it would perhaps make more sense to just use RGB instead
of RGBA, seeing as the alpha channel is permanently set to be opaque.

Change-Id: I86ad758c6a8bee21b265730727a76605e5850c0c

desktop/qa/gtktiledviewer/gtktiledviewer.cxx

@@ -75,11 +75,6 @@ bool drawCallback(GtkWidget* /* The eventbox */, void* /* cairo_t* cr */, gpoint
     // wrong.
     (void) nRowStride;
 
-    for (int i = 3; i < nRowStride*nRenderHeight; i += 4)
-    {
-        pBuffer[i] = 0xFF;
-    }
-
     gtk_image_set_from_pixbuf( GTK_IMAGE( ourCanvas ), ourPixBuf );
 
     return true;