Fix input race causing tooltip flicker and unresponsive keyboard

[HugeFrog24]

Two bugs, one cause.

1. Tooltips flickered every few seconds while a finger was held still, most obvious with Limit FPS on.
2. Tapping a text field sometimes only moved the caret without opening the keyboard, until you tapped somewhere else first.

Both came down to Canvas touching ImGui from two threads at once, the render thread and the Android UI thread. Input from the UI thread is now buffered and replayed on the render thread, so ImGui state is only ever written from one place.

Verified against libSPT, libTSM and libtibik on a Galaxy A52 and a OnePlus 13: tooltips stable, keyboard reliable, scrolling smooth, existing mod behaviour unchanged.

Happy to expand on any of it.

Before / after

Tooltip flicker

Before:

flicker-before.mp4

After:

flicker-after.mp4

Keyboard not opening

Before:

keyboard-before.mp4

After:

keyboard-after.mp4