src/xvba_video_glx.c: fix refcounting surface.

A GLX surface can be in use by multiple VASurfaces, therefor make
sure refcounting is done properly. The code did not honor this and
could destroy a surface that was still in use by another GLX Surface
object.

The code is changed to enforce proper refcounting. Basically using

   destroy_glx_surface(driver_data, obj_glx_surface);

is suspicious and should be replaced by:

   glx_surface_unref(driver_data, obj_glx_surface);

instead. This function decreases the refcount on obj_glx_surface and
when it reaches 0, then it destroys the surface properly.

src/xvba_video_glx.c

@@ -792,6 +792,7 @@ end:
     glDisable(target);
 
     if (is_error && obj_glx_surface) {
+        obj_glx_surface->refcount = 0;
         destroy_glx_surface(driver_data, obj_glx_surface);
         obj_glx_surface = NULL;
     }
@@ -1598,7 +1599,7 @@ xvba_DestroySurfaceGLX(
     if (!gl_set_current_context(new_cs, &old_cs))
         return VA_STATUS_ERROR_OPERATION_FAILED;
 
-    destroy_glx_surface(driver_data, obj_glx_surface);
+    glx_surface_unref(driver_data, obj_glx_surface);
 
     gl_destroy_context(new_cs);
     gl_set_current_context(&old_cs, NULL);
@@ -1807,7 +1808,7 @@ glx_output_surface_destroy(
 
     if (obj_output->gl_surface) {
         if (!obj_output->parent)
-            destroy_glx_surface(driver_data, obj_output->gl_surface);
+            glx_surface_unref(driver_data, obj_output->gl_surface);
         obj_output->gl_surface = NULL;
     }
 
@@ -2093,7 +2094,7 @@ glx_output_surface_ensure_size(
 
     /* Make sure the VA/GLX surface is created */
     if (size_changed) {
-        destroy_glx_surface(driver_data, glx_output->gl_surface);
+        glx_surface_unref(driver_data, glx_output->gl_surface);
         glx_output->gl_surface = NULL;
     }
     if (!glx_output->gl_surface) {