fix: full repaint on resume to fix transparency artifacts in Kitty

[acegikmoo]

Fixes #922

When Kitty resumes after SIGCONT, it restores the alternate screen buffer with transparency compositing applied. The diff render then skips cells that haven't changed, so the transparency persists since nothing in the UI actually changed during suspension.

The fix is to call lib.render with force=true right after currentRenderBuffer.clear() in resume(). This bypasses the diff and repaints every cell immediately, overwriting whatever Kitty had in its restored buffer.

This is also why resizing fixes it, buffer.resize() clears both buffers which makes the diff see everything as dirty and forces a full repaint as a side effect.

this.currentRenderBuffer.clear(this.backgroundColor)
// Normal diff skips unchanged cells leaving transparency visible. force=true repaints all.
this.lib.render(this.rendererPtr, true)

[kommander]

this.currentRenderBuffer.clear(this.backgroundColor) was supposed to do that, but I guess behaviour might have changed recently when this.backgroundColor has alpha=0 it does not reset the cells. When we do a force re-render it will draw all cells and the currentRenderBuffer.clear becomes futile, so we would not need to do both.

I wonder if that is easily coverable with a test maybe, so it doesn't regress again 🤔

[acegikmoo]

this.currentRenderBuffer.clear(this.backgroundColor) was supposed to do that, but I guess behaviour might have changed recently when this.backgroundColor has alpha=0 it does not reset the cells. When we do a force re-render it will draw all cells and the currentRenderBuffer.clear becomes futile, so we would not need to do both.

I wonder if that is easily coverable with a test maybe, so it doesn't regress again 🤔

removed the clear() call since the force render covers it. Also added a test that spies on lib.render and asserts force=true is passed on resume, so this should be covered against regression ig