stdinc: On Windows, uppercase the system environment table's var names.

Fixes #15086.

include/SDL3/SDL_stdinc.h

@@ -1776,6 +1776,12 @@ typedef struct SDL_Environment SDL_Environment;
  * SDL_setenv_unsafe() or SDL_unsetenv_unsafe() if you want changes to persist
  * in the C runtime environment after SDL_Quit().
  *
+ * Note that on Windows, the variable names pulled in from the system at
+ * startup have their ASCII values uppercased, to match what most platforms
+ * expect even though the Windows system environment table is
+ * case-insensitive. Once those uppercased variable names are in an
+ * SDL_Environment, SDL treats them as case-sensitive.
+ *
  * \returns a pointer to the environment for the process or NULL on failure;
  *          call SDL_GetError() for more information.
  *
@@ -1791,7 +1797,13 @@ typedef struct SDL_Environment SDL_Environment;
 extern SDL_DECLSPEC SDL_Environment * SDLCALL SDL_GetEnvironment(void);
 
 /**
- * Create a set of environment variables
+ * Create a set of environment variables.
+ *
+ * Note that on Windows, the variable names pulled in from the system, if
+ * `populated` is true, have their ASCII values uppercased, to match what most
+ * platforms expect even though the Windows system environment table is
+ * case-insensitive. Once those uppercased variable names are in an
+ * SDL_Environment, SDL treats them as case-sensitive.
  *
  * \param populated true to initialize it from the C runtime environment,
  *                  false to create an empty environment.

src/stdlib/SDL_getenv.c

@@ -119,6 +119,11 @@ SDL_Environment *SDL_CreateEnvironment(bool populated)
                 }
                 *value++ = '\0';
 
+                // uppercase ASCII chars in environment variable names on Windows, since the system environment table is case-insensitive.
+                for (char *ptr = variable; *ptr; ptr++) {
+                    *ptr = (char) SDL_toupper((int) *ptr);  // the only UTF-8 bytes that don't have the high-bit set are single-byte ASCII chars, so SDL_toupper will leave multichar stuff alone.
+                }
+
                 SDL_InsertIntoHashTable(env->strings, variable, value, true);
             }
             FreeEnvironmentStringsW(strings);
@@ -143,6 +148,13 @@ SDL_Environment *SDL_CreateEnvironment(bool populated)
                 }
                 *value++ = '\0';
 
+#ifdef SDL_PLATFORM_CYGWIN
+                // uppercase ASCII chars in environment variable names on Windows, since the system environment table is case-insensitive.
+                for (char *ptr = variable; *ptr; ptr++) {
+                    *ptr = (char) SDL_toupper((int) *ptr);  // the only UTF-8 bytes that don't have the high-bit set are single-byte ASCII chars, so SDL_toupper will leave multichar stuff alone.
+                }
+#endif
+
                 SDL_InsertIntoHashTable(env->strings, variable, value, true);
             }
         }

test/testprocess.c

@@ -197,8 +197,8 @@ static int SDLCALL process_testInheritedEnv(void *arg)
     int exit_code;
     char random_env1[64];
     char random_env2[64];
-    static const char *const TEST_ENV_KEY1 = "testprocess_inherited_var";
-    static const char *const TEST_ENV_KEY2 = "testprocess_other_var";
+    static const char *const TEST_ENV_KEY1 = "TESTPROCESS_INHERITED_VAR";
+    static const char *const TEST_ENV_KEY2 = "TESTPROCESS_OTHER_VAR";
     char *test_env_val1 = NULL;
     char *test_env_val2 = NULL;
     char *buffer = NULL;
@@ -269,8 +269,8 @@ static int SDLCALL process_testNewEnv(void *arg)
     int exit_code;
     char random_env1[64];
     char random_env2[64];
-    static const char *const TEST_ENV_KEY1 = "testprocess_inherited_var";
-    static const char *const TEST_ENV_KEY2 = "testprocess_other_var";
+    static const char *const TEST_ENV_KEY1 = "TESTPROCESS_INHERITED_VAR";
+    static const char *const TEST_ENV_KEY2 = "TESTPROCESS_OTHER_VAR";
     char *test_env_val1 = NULL;
     char *test_env_val2 = NULL;
     char *buffer = NULL;