Fix display_name returning index instead of name on macOS

[ralphweng2023]

Description

This PR fixes the display_name() function returning display index numbers ("0", "1", "2") instead of actual display names on macOS.

Problem

On newer macOS versions, SDL_GetDisplayName() returns the display index as a string instead of the actual display name. This is a known SDL2 limitation.

Before:

Display Number: 1
Name: 0

After:

Display Number: 1
Name: Built-in Retina Display

Root Cause

SDL_GetDisplayName() in SDL2 does not properly retrieve display names on macOS 10.15+ (Catalina and later).

Solution

Use native macOS NSScreen.localizedName API via Objective-C++ to get proper display names. The implementation:

1. Adds a new core_driver_macos.mm file with macOS-specific code
2. Uses NSScreen.localizedName which returns proper names like "Built-in Retina Display" or "DELL U2720Q"
3. Falls back to SDL_GetDisplayName() on older macOS versions (pre-10.15) or other platforms

Files Changed

• core_driver.cpp: Added macOS-specific code path using sk_macos_get_display_name()
• core_driver_macos.mm: New Objective-C++ file implementing the macOS NSScreen API
• CMakeLists.txt: Updated to include .mm files in the macOS build

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

1. Created standalone test program using NSScreen.localizedName:

$ /tmp/test_macos_display
Number of screens: 1
Display 0: 'Built-in Retina Display'

2. Built SplashKit on macOS with the fix:

cd projects/cmake
cmake --preset macOS
cmake --build build/
# Build succeeded

Testing Checklist

• Code compiles without errors on macOS
• Native macOS API returns correct display name
• Falls back gracefully on older macOS versions
• Tested on Linux (no changes to Linux path)
• Tested on Windows (no changes to Windows path)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes