renderer/opengl: acquire lock to prep kitty images

Fixes #1101

GTK forces all GLArea drawing to happen on the GUI thread. In this
scenario, `updateFrame` and `drawFrame` could be happening in parallel.
This commit grabs the lock while updating the Kitty image protocol state
since that is used by both functions.

We already have a mutex called "draw_mutex" that we hold while we're
modifying data that could be shared by this separate draw thread. We
should reuse that here.

src/renderer/OpenGL.zig

@@ -709,6 +709,11 @@ pub fn updateFrame(
         // We only do this if the Kitty image state is dirty meaning only if
         // it changes.
         if (state.terminal.screen.kitty_images.dirty) {
+            // prepKittyGraphics touches self.images which is also used
+            // in drawFrame so if we're drawing on a separate thread we need
+            // to lock this.
+            if (single_threaded_draw) self.draw_mutex.lock();
+            defer if (single_threaded_draw) self.draw_mutex.unlock();
             try self.prepKittyGraphics(state.terminal);
         }