sdl: Fix display list damage tracking

[TomHoenderdos]

Split out of #18.

The SDL display only redraws where the new display list differs from the previous one. The code computing that damage area had several bugs:

• cmp_display_item() compared scaled_cropped_image items through data.image_data.pix, which overlaps the width and height of data.image_data_with_size, not its pixel pointer. Two items showing different images of the same size compared equal and the change was never drawn.
• The diff compared each new item with orig[j] without checking j, so a new list longer than the previous one read past the end of the previous item array.
• Skipped old items were damaged from index k - j instead of j, so removing the first item of a list damaged nothing.
• Old items after the last match were never damaged, so removing the last item or moving an item left its old pixels on screen.
• update_damaged_area() and damage_clip() overwrote x and y before computing the right and bottom edges from them, so merging a rectangle left of or above the current damage shrank the area, and clipping one that started off screen kept its full size.

display.c still redraws the whole screen after computing the damage (BUG: damage area is not correct), so only the out-of-bounds read had a visible effect. That workaround stays until the SDL plugin builds again and the damage area can be checked on screen.

Commits

1. sdl: Move display list damage tracking to its own file: moves the comparison and damage rectangle helpers, which don't use SDL, from display.c to damage.c so they can be tested on their own. Unchanged apart from the two functions display.c calls, now damage_diff() and damage_clip().
2. sdl: Fix display list damage comparison: the first two bugs, plus tests/damage, a host test under ASan and UBSan.
3. sdl: Fix damage areas of removed and moved items: the other three, with tests.

Testing

tests/damage against AtomVM release-0.7, each commit's tests run against the code before it:

	before	after
commit 2	other image of the same size: no damage; item added at the end: ASan heap-buffer-overflow	3 of 3 pass
commit 3	first, middle and last items removed, item moved, clip at the top left: wrong damage (5 of 9 fail)	9 of 9 pass

cmake -S tests/damage -B build/damage -DLIBATOMVM_INCLUDE_PATH=<AtomVM>/src/libAtomVM
cmake --build build/damage && build/damage/test_damage

The SDL plugin itself isn't built: on main it already fails to compile against SDL2 and the current ufont_manager_register(). display.c has the same compiler errors before and after this PR, and damage.c builds without warnings.

🤖 Generated with Claude Code

[bettio]

If you don't mind I would rather close this PR and merge instead #23 that goes into the direction of handling partial updates. The rest of your open PR should be unaffected.