libdmview: draw nothing beneath an opaque box of the root

[JohnAmadis]

Summary

A redraw of the root used to draw everything, including what an opaque box drawn later hides. The usual case is an application window sliding over the home screen: the whole home screen was drawn on every animation frame, and then the window over most of it.

Now, before a redraw of the root:

1. The view runs once without drawing (an empty clip). That finds where the boxes are this time, since a box may move with a variable ($win_y).
2. The occluder is chosen from the opaque boxes of the root (OPAQUE, not translucent, visible): the one that hides the most of the redrawn area, as long as the rest is one rectangle. An example is a full-width box at the top or bottom of the area.
3. In the real run, everything executed before the occluder's BOX is drawn only in the part that shows, and from that BOX on the full clip is back. Execution order matters, not position in the code, so subroutines called before the window are covered.

Views without opaque boxes of the root do not pay for the extra run. Partial redraws of a box are unchanged.

Testing

• Same pixels: frames of the dmodOS example rendered on the host in RGB565, before and after the change, are byte-for-byte identical. That covers 9 frames, including the opening and closing animations at several points.
• Tests: test_libdmview passes.
• STM32F746G-DISCO: rendering a full-screen frame of the slide animation (DWT cycle counter) takes 13–24 ms, down from 40–47 ms. That includes the run without drawing (~0.5 ms).

🤖 Generated with Claude Code