Fix signed integer overflow in GetCurrentMonitor distance calc

[Vasilis-Narain]

Problem

GetCurrentMonitor() in rcore_desktop_glfw.c computes int dist = dx*dx + dy*dy where dx, dy are pixel offsets from window centre to monitor corner. On my setup (multi-monitor WSL2 through Windows 11) the squared sum exceeds INT_MAX, causing signed integer overflow UB. Today GCC wraps silently, but UBSan-instrumented builds (eg. Zig or clang with -fsanitize=undefined) fail.

Environment

• OS: Ubuntu 24.04.1 on WSL2 (Windows 11 host), X11 via WSLg
• Compiler: gcc 13.3.0
• Display 1920x1200
• Detected on: Zig 0.16 bundling raylib 6.0

Repro

Originally caught by Zig 0.16's UBSan trapping on the bundled raylib build:

thread panic: signed integer overflow: 1173062500 + 1050343281 cannot be represented in type 'int'
rcore_desktop_glfw.c:887

Reproduced standalone by adding a printf next to the overflow site and running examples/core/core_basic_window:

REPRO i=0 dx=-34250 dy=-32484 prod=2228272756 INT_MAX=2147483647 overflow=1
REPRO i=1 dx=-32330 dy=-32484 prod=2100439156 INT_MAX=2147483647 overflow=0

prod (computed as (long long)dx*dx + (long long)dy*dy) exceeds INT_MAX, confirming the truncation.

Fix

Promote dist and closestDist to long long. No behavior change for monitors within INT_MAX distance and correctly handles cases beyond it without UB.