Since context_acquire() doesn't take the resource location into account, it
may consider the back-buffer to be offscreen, and return an already active
context for a different swapchain. This fixes a regression introduced by
commit 64d41b3d.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>