shared-module/busdisplay: merge overlapping dirty rectangles

[lynt-smitka]

Addresses #10687: when dirty rectangles overlap, displayio resends the shared pixels once per rectangle (and draws them as separate, visible passes). A changing text label is the common trigger.

@tannewt proposed two options in the issue - a quick heuristic (sum the dirty areas; if they exceed the screen, do a full refresh) or the more complex rectangle merge. This PR implements merge. The heuristic only helps when the summed dirty area exceeds the whole screen; it does nothing when the change covers only part of the screen - the common case, e.g. a label that updates but doesn't fill the display. Merge helps there too, and when the change does fill the screen it matches the heuristic (the union is then the whole screen anyway).

Measured on a PicoPad (ST7789 SPI)

scene	baseline [fps]	heuristic (sum>screen) [fps]	merge [fps]
large sub-screen text (scale 8)	10.45	10.07 (never triggers)	19.60
giant text, screen-filling (scale 20, reported case)	1.75	3.30	3.30
parallax (full-screen bg + band)	1.64	2.42	2.42

What it does

Before each refresh, the clipped dirty areas are copied into a small scratch array and any pair whose bounding box is smaller than the two summed is fused; non-overlapping rectangles stay separate. The motivating case is a changing text label: it dirties a whole-label rectangle plus a nested per-glyph rectangle per character, sending those pixels twice - merge collapses them into one.

• Never sends more pixels than the unmerged code - no regression.
• Greedy, bounded by MAX_MERGE_AREAS (16); above that, refreshes the raw list unmerged.
• Near-zero for a single moving sprite (nothing to merge).
• +304 B flash, ~192 B stack, no heap.

Fixes #10687

[tannewt]

What do you think about having Group do this instead? It'll have more context and work for all different display types.

[lynt-smitka]

What do you think about having Group do this instead? It'll have more context and work for all different display types.

I agree, that will be more versatile. I'll see where it would be best to place it (shared-module/displayio/area.c?) and try to move it there.

[lynt-smitka]

Moved the merge into shared displayio code (area.c + display_core), used by busdisplay, framebufferio and epaperdisplay.

Group could still win a different case - combining a moved group's areas at the source (they touch rather than overlap, so list-level merging can't). But that's a bigger change. Should I try to implement it in this PR or make a new one?

[tannewt]

Please do it in group to prevent duplication. The comment has a link to where I'd do it.

[tannewt]

This needs to be repeated because the areas are const by this time. Instead do it here:

circuitpython/shared-module/displayio/Group.c

Lines 438 to 471 in 43a7b49

	displayio_area_t *displayio_group_get_refresh_areas(displayio_group_t *self, displayio_area_t *tail) {
	if (self->item_removed) {
	self->dirty_area.next = tail;
	tail = &self->dirty_area;
	}
	for (int32_t i = self->members->len - 1; i >= 0; i--) {
	mp_obj_t layer;
	#if CIRCUITPY_VECTORIO
	const vectorio_draw_protocol_t *draw_protocol = mp_proto_get(MP_QSTR_protocol_draw, self->members->items[i]);
	if (draw_protocol != NULL) {
	layer = draw_protocol->draw_get_protocol_self(self->members->items[i]);
	tail = draw_protocol->draw_protocol_impl->draw_get_refresh_areas(layer, tail);
	continue;
	}
	#endif
	layer = mp_obj_cast_to_native_base(
	self->members->items[i], &displayio_tilegrid_type);
	if (layer != MP_OBJ_NULL) {
	if (!displayio_tilegrid_get_rendered_hidden(layer)) {
	tail = displayio_tilegrid_get_refresh_areas(layer, tail);
	}
	continue;
	}
	layer = mp_obj_cast_to_native_base(
	self->members->items[i], &displayio_group_type);
	if (layer != MP_OBJ_NULL) {
	tail = displayio_group_get_refresh_areas(layer, tail);
	continue;
	}
	}
	return tail;
	}

There is always a root group that will do the top level. That way displays don't need to worry about it at all.