Adopt CSS3 cursor naming throughout

[Copilot]

Closes #4522

What's new?

Mir cursor naming mixed CSS3 and xcursor conventions inconsistently. The codebase now uses CSS3 cursor names throughout while maintaining xcursor theme compatibility.

Added CSS3 cursor constants:

• mir_diagonal_resize_top_to_left_cursor_name = "nw-resize"
• mir_diagonal_resize_bottom_to_left_cursor_name = "sw-resize"

Eliminated hardcoded xcursor names:

// Before
case mir_resize_edge_southwest:
    cursor_name = "bottom_left_corner";  // xcursor name
    break;

// After
case mir_resize_edge_southwest:
    cursor_name = mir_diagonal_resize_bottom_to_left_cursor_name;  // CSS3 name
    break;

Compatibility layer:

• Extended xcursor_name_for_mir_cursor() to map CSS3 → xcursor theme names
• Added CSS3 aliases to builtin cursor theme for fallback scenarios

How to test

Run miral-app with or without --cursor-theme:

• Resize window corners to verify all 8 directional cursors appear correctly
• Test works with both XCursorLoader (themed) and BuiltinCursorImages (fallback)

Checklist

• Tests added and pass
• Adequate documentation added
• (optional) Added Screenshots or videos

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Cursor naming is an ad-hoc mix of "standards"</issue_title>
<issue_description>We have some cursor names that claim to be based on https://www.w3.org/TR/css-ui-3/#propdef-cursor:

mir/src/common/mir_cursor_api.cpp

Line 19 in 14f1cfe

// Cursor names are from CSS3: https://www.w3.org/TR/css-ui-3/#propdef-cursor

But these are neither complete, nor named according to the CSS reference.

We also try to map these to "xcursor" naming but there is no definitive list of xcursor names and current practice is to use the CSS list mentioned above.

We should probably adopt the CSS cursors throughout.

See also: #4522, and

mir/src/server/shell/decoration/input.cpp

Lines 563 to 564 in 14f1cfe

	case mir_resize_edge_southwest:
	cursor_name = "bottom_left_corner";

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Cursor naming is an ad-hoc mix of "standards" #4524

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.