libva: render front buffer for Pixmap.

The way to get drawable type is proposed by Gwenole Beauchesne <gbeauchesne@splitted-desktop.com>

va/x11/dri2_util.c

@@ -98,7 +98,11 @@ dri2GetRenderingBuffer(VADriverContextP ctx, struct dri_drawable *dri_drawable)
     VA_DRI2Buffer *buffers;
     
     i = 0;
-    attachments[i++] = __DRI_BUFFER_BACK_LEFT;
+    if (dri_drawable->is_window)
+        attachments[i++] = __DRI_BUFFER_BACK_LEFT;
+    else
+        attachments[i++] = __DRI_BUFFER_FRONT_LEFT;
+
     buffers = VA_DRI2GetBuffers(ctx->native_dpy, dri_drawable->x_drawable,
 			     &dri2_drawable->width, &dri2_drawable->height, 
                              attachments, i, &count);

va/x11/va_dricommon.c

 #include "va_dricommon.h"
 
-static struct dri_drawable *                                                                                                                                                                                   
+// X error trap
+static int x11_error_code = 0;
+static int (*old_error_handler)(Display *, XErrorEvent *);
+
+static int 
+error_handler(Display *dpy, XErrorEvent *error)
+{
+    x11_error_code = error->error_code;
+    return 0;
+}
+
+static void 
+x11_trap_errors(void)
+{
+    x11_error_code    = 0;
+    old_error_handler = XSetErrorHandler(error_handler);
+}
+
+static int 
+x11_untrap_errors(void)
+{
+    XSetErrorHandler(old_error_handler);
+    return x11_error_code;
+}
+
+static int 
+is_window(Display *dpy, Drawable drawable)
+{
+    XWindowAttributes wattr;
+
+    x11_trap_errors();
+    XGetWindowAttributes(dpy, drawable, &wattr);
+    return x11_untrap_errors() == 0;
+}
+
+static struct dri_drawable *
 do_drawable_hash(VADriverContextP ctx, XID drawable)
 {
     struct dri_state *dri_state = (struct dri_state *)ctx->dri_state;
@@ -15,6 +50,7 @@ do_drawable_hash(VADriverContextP ctx, XID drawable)
 
     dri_drawable = dri_state->createDrawable(ctx, drawable);
     dri_drawable->x_drawable = drawable;
+    dri_drawable->is_window = is_window((Display *)ctx->native_dpy, drawable);
     dri_drawable->next = dri_state->drawable_hash[index];
     dri_state->drawable_hash[index] = dri_drawable;
 

va/x11/va_dricommon.h

@@ -40,6 +40,7 @@ union dri_buffer
 struct dri_drawable 
 {
     XID x_drawable;
+    int is_window;
     int x;
     int y;
     unsigned int width;