sdl2-compat: Restore comment explaining why volatile is used in SDL2Compat_strlen()

From ac2da24150a969860de50ec819776cabe7e8df2a Mon Sep 17 00:00:00 2001
From: Simon McVittie <[EMAIL REDACTED]>
Date: Mon, 15 Jun 2026 12:20:34 +0100
Subject: [PATCH] Restore comment explaining why volatile is used in
 SDL2Compat_strlen()

Signed-off-by: Simon McVittie <smcv@collabora.com>
---
 src/sdl2_compat.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/sdl2_compat.c b/src/sdl2_compat.c
index 550672c9..8f0db91a 100644
--- a/src/sdl2_compat.c
+++ b/src/sdl2_compat.c
@@ -272,6 +272,9 @@ SDL2COMPAT_itoa(char *dst, int val)
 /* you can use SDL3_strlen once we're past startup. */
 static int SDL2Compat_strlen(const char *str)
 {
+    /* volatile prevents gcc from optimizing this into a call to the
+     * strlen() library function, which we are intentionally avoiding on
+     * Windows: see #340 */
     volatile const char *ptr = str;
     while (*ptr) {
         ++ptr;