vulkan/wsi: Set X11 minImageCount to 3.

For IMMEDIATE and FIFO, most games work in a pipelined manner where the
can produce frames at a rate of 1/MAX(CPU duration, GPU duration), but
the render latency is CPU duration + GPU duration.

This means that with scanout from pageflipping we need 3 frames to run
full speed:
1) CPU rendering work
2) GPU rendering work
3) scanout

Once we have a nonblocking acquire that returns a semaphore we can merge
1 and 3. Hence the ideal implementation needs only 2 images, but games
cannot tellwe currently do not have an ideal implementation and that
hence they need to allocate 3 images. So let us do it for them.

This is a tradeoff as it uses more memory than needed for non-fullscreen
and non-performance intensive applications.

Since this is pretty much a TODO that can use the context I added this as
a comment.

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/vulkan/wsi/wsi_common_x11.c

@@ -501,13 +501,25 @@ x11_surface_get_capabilities(VkIcdSurfaceBase *icd_surface,
                                       VK_COMPOSITE_ALPHA_OPAQUE_BIT_KHR;
    }
 
-   /* For true mailbox mode, we need at least 4 images:
-    *  1) One to scan out from
-    *  2) One to have queued for scan-out
-    *  3) One to be currently held by the X server
-    *  4) One to render to
+   /* For IMMEDIATE and FIFO, most games work in a pipelined manner where the
+    * can produce frames at a rate of 1/MAX(CPU duration, GPU duration), but
+    * the render latency is CPU duration + GPU duration.
+    *
+    * This means that with scanout from pageflipping we need 3 frames to run
+    * full speed:
+    * 1) CPU rendering work
+    * 2) GPU rendering work
+    * 3) scanout
+    *
+    * Once we have a nonblocking acquire that returns a semaphore we can merge
+    * 1 and 3. Hence the ideal implementation needs only 2 images, but games
+    * cannot tellwe currently do not have an ideal implementation and that
+    * hence they need to allocate 3 images. So let us do it for them.
+    *
+    * This is a tradeoff as it uses more memory than needed for non-fullscreen
+    * and non-performance intensive applications.
     */
-   caps->minImageCount = 2;
+   caps->minImageCount = 3;
    /* There is no real maximum */
    caps->maxImageCount = 0;