feat: screenshot api

[nborodikhin]

Background

I want to use a programmatic input driver in my app (see pr: nborodikhin/nextui-music-player#35) to be able to run scripts like this:

hold(L1, keep)
press(DOWN), press(DOWN)
screenshot(shots/with-l1-down.png)
release(L1)

I found a way to inject button events, but I need a NextUI API to take the screenshot, since the full screen is composed internally by NextUI.

Change

• add PLAT_captureScreenshot() api
• implement composition (same as PLAT_Flip()) preserving renderer state

Before (main texture only)	After (full composition)