wsi/x11: Hook up KHR_incremental_present

We create one XFIXES region per swapchain image. If the QueuePresent comes in with a list of rectangles, we push them into the region and pass it to xcb_present_pixmap.

The extension is technically just a hint. We still fall back to the unhinted "update the whole image" path if the update region has more than an arbitrary number of rects, or if we're stuck using plain PutImage instead of ShmPutImage.