[client] overlay: coalesce pointer redraws during splash fade

Pointer updates bypassed the paced overlay render and woke the renderer
for every cursor packet. On the default single-buffer EGL surface,
each full fade redraw could expose the desktop before the splash was
blended again.

Coalesce pointer redraws into the existing fade cadence while keeping
the latest cursor state. Immediate cursor redraw resumes when the fade
ends.

client/src/overlays.h

@@ -122,6 +122,7 @@ typedef enum LG_UserStatus
 LGUserStatus;
 
 void overlaySplash_show(bool show);
+bool overlaySplash_isFading(void);
 void overlayStatus_set(LGUserStatus, bool value);
 
 #endif