st/glx: fix depth/stencil format selection code

This fixes a pretty big performance regression caused by commit
3475e88442.

When the user does not request a stencil buffer it's important that we
don't use a depth/stencil format (or at least make it our last choice).
If the user calls glClear(GL_DEPTH_BUFFER_BIT) when we have a combined
depth/stencil buffer, that causes us to hit the clear_with_quad() path
which can be much, much slower than calling pipe_context::clear().

Also, try to use a shallower depth format before a deeper one.

src/gallium/state_trackers/glx/xlib/xm_api.c

@@ -309,8 +309,9 @@ choose_pixel_format(XMesaVisual v)
    return 0;
 }
 
+
 /**
- * Choose a depth/stencil format that is "better" than the given depth and
+ * Choose a depth/stencil format that satisfies the given depth and
  * stencil sizes.
  */
 static enum pipe_format
@@ -324,16 +325,20 @@ choose_depth_stencil_format(XMesaDisplay xmdpy, int depth, int stencil)
    int count, i;
 
    count = 0;
+
+   if (depth <= 16 && stencil == 0) {
+      formats[count++] = PIPE_FORMAT_Z16_UNORM;
+   }
+   if (depth <= 24 && stencil == 0) {
+      formats[count++] = PIPE_FORMAT_X8Z24_UNORM;
+      formats[count++] = PIPE_FORMAT_Z24X8_UNORM;
+   }
    if (depth <= 24 && stencil <= 8) {
       formats[count++] = PIPE_FORMAT_S8Z24_UNORM;
       formats[count++] = PIPE_FORMAT_Z24S8_UNORM;
    }
-
-   if (!stencil) {
-      if (depth <= 16)
-         formats[count++] = PIPE_FORMAT_Z16_UNORM;
-      if (depth <= 32)
-         formats[count++] = PIPE_FORMAT_Z32_UNORM;
+   if (depth <= 32 && stencil == 0) {
+      formats[count++] = PIPE_FORMAT_Z32_UNORM;
    }
 
    fmt = PIPE_FORMAT_NONE;