screen / framebuffer widths which are not a multiple of 64 + Present extension + using modifiers is causing present-flip errors

The root-cause of this is likely a mesa issue and I've also filed a mesa bug for this, see: https://bugs.freedesktop.org/show_bug.cgi?id=111306

I'm still filing an xserver bug for this because:

1. This is a regression compared to 1.20, triggered by enabling modifier support in glamor
2. This may require some work on the xserver side too
3. This can be worked around on the xserver side

The problem is that when the user ends up with a screen / framebuffer width which is not a multiple of 64 and Present flipping is used, the drmModeAddFB2WithModifiers calls done to create a drmfb to flip fail and the log fills with these errors:

[ 28843.414] (WW) modeset(0): Page flip failed: Invalid argument
[ 28843.414] (EE) modeset(0): present flip failed
[ 28843.595] (WW) modeset(0): Page flip failed: Invalid argument
[ 28843.595] (EE) modeset(0): present flip failed

Reproduction instructions:

1. Take a Skylake iGPU (in my case with 2 1920x1080 monitors attached)
2. Run Xorg master with the modesetting driver (master is necessary because glamor has support for modifiers enabled only in master)
3. Run a recent gnome-shell on top of Xorg
4. Run xrandr --fb x changing the width to be 32 pixels larger then necessary (or any other value not a multiple of 64)
5. Watch Xorg.log filling with errors like these:

A quick and dirty xserver side workaround is this:

diff --git a/hw/xfree
86/modes/xf86RandR12.c b/hw/xfree86/modes/xf86RandR12.c
index b476c07ce..846a2325a 100644
--- a/hw/xfree86/modes/xf86RandR12.c
+++ b/hw/xfree86/modes/xf86RandR12.c
@@ -693,6 +693,12 @@ xf86RandR12ScreenSetSize(ScreenPtr pScreen,
     if (pRoot && pScrn->vtSema)
         (*pScrn->EnableDisableFBAccess) (pScrn, FALSE);
 
+    /*
+     * Present flipping with modifiers may fail if our screen width is not
+     * a multiple of 64.
+     */
+    width = (width + 63) & ~63;
+
     /* Let the driver update virtualX and virtualY */
     if (!(*config->funcs->resize) (pScrn, width, height))
         goto finish;

Which works surprisingly well, so we may even consider actually going with this as a workaround.