x11: Make damage region fetching code more robust to errors

With DamageReportNonEmpty damage report level, the x server will
send kwin a DamageNotify when the damage region changes from empty to
not empty.

The damage region will be made empty when SurfaceItemX11 calls
xcb_damage_subtract().

It appears like xcb_generate_id() can return us an already associated
XID, which eventually results in xcb_damage_subtract() failing and
breaking state tracking in SurfaceItemX11. KWin will no longer receive
DamageNotify events and some windows will freeze.

In order to make getting BadIdChoice less catastrophic, this change
makes the SurfaceItemX11 reset m_isDamaged after successfully fetching
the damage region. If xcb_generate_id() returns us a bad id, kwin will
try to fetch the damage again in the next frame.

BUG: 456511

src/surfaceitem_x11.cpp

@@ -54,7 +54,6 @@ bool SurfaceItemX11::fetchDamage()
     if (!m_isDamaged) {
         return false;
     }
-    m_isDamaged = false;
 
     if (m_damageHandle == XCB_NONE) {
         return true;
@@ -105,6 +104,7 @@ void SurfaceItemX11::waitForDamage()
     free(reply);
 
     addDamage(region);
+    m_isDamaged = false;
 }
 
 void SurfaceItemX11::destroyDamage()